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Biological Sciences Faculty Publications
Biological Sciences
6-2000
A Biochemical Mechanism for Nonrandom
Mutations and Evolution
Barbara E. Wright
University of Montana - Missoula, [email protected]
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Wright, Barbara E., "A Biochemical Mechanism for Nonrandom Mutations and Evolution" (2000). Biological Sciences Faculty
Publications. Paper 263.
http://scholarworks.umt.edu/biosci_pubs/263
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JOURNAL OF BACTERIOLOGY, June 2000, p. 2993–3001
0021-9193/00/$04.00⫹0
Copyright © 2000, American Society for Microbiology. All Rights Reserved.
Vol. 182, No. 11
MINIREVIEW
A Biochemical Mechanism for Nonrandom
Mutations and Evolution
BARBARA E. WRIGHT*
nario begins with the starvation of a self-replicating unit for its
precursor, metabolite A, utilized by enzyme 1 encoded by gene
1. When metabolite A is depleted, a mutation in a copy of gene
1 gives rise to gene 2 and allows enzyme 2 to use metabolite B
by converting it to metabolite A. Then metabolite B is depleted, obtained from metabolite C, and so on, as an increasingly complex biochemical pathway evolves. In fact, there are
examples in which a similar series of events can actually be
observed in the laboratory, for example, involving enzymes
that are “borrowed” from existing pathways, via regulatory
mutations, to establish new pathways (75).
The starvation conditions that may initiate a series of events
such as those described above target the most relevant genes
for increased rates of transcription, which in turn increase rates
of mutation (111). Transcriptional activation can result from
the addition of a substrate or from the removal of a repressor
or an end product inhibitor. The latter mechanism, called
derepression, occurs in response to starvation for an essential
substrate or for an end product that represses its own synthesis
by feedback inhibition. Since evolution usually occurs in response to stress (41), transcriptional activation via derepression is the main focus of this minireview.
As this minireview is concerned with the importance of the
environment in directing evolution, it is appropriate to remember that Lamarck was the first to clearly articulate a consistent
theory of gradual evolution from the simplest of species to the
most complex, culminating in the origin of mankind (71). He
published his remarkable and courageous theory in 1809, the
year of Darwin’s birth. Unfortunately, Lamarck’s major contributions have been overshadowed by his views on the inheritance of acquired characters. In fact, Darwin shared some of
these same views, and even Weismann (106), the father of
neo-Darwinism, decided late in his career that directed variation
must be invoked to understand some phenomena, as random
variation and selection alone are not a sufficient explanation
(71). This minireview will describe mechanisms of mutation
that are not random and can accelerate the process of evolution in specific directions. The existence of such mechanisms
has been predicted by mathematicians (6) who argue that, if
every mutation were really random and had to be tested
against the environment for selection or rejection, there would
not have been enough time to evolve the extremely complex
biochemical networks and regulatory mechanisms found in
organisms today. Dobzhansky (21) expressed similar views by
stating “The most serious objection to the modern theory of
evolution is that since mutations occur by ‘chance’ and are
undirected, it is difficult to see how mutation and selection can
add up to the formation of such beautifully balanced organs as,
for example, the human eye.”
The most primitive kinds of cells, called progenotes by
Woese (108), were undoubtedly very simple biochemically with
only a few central anabolic and catabolic pathways. Wächterhäuser (103) theorizes that the earliest metabolic pathway was
a reductive citric acid cycle by which carbon fixation occurred
(64). At that point in time, some four billion years ago, how did
the additional, more complex metabolic pathways found in
even the simplest prokaryotes evolve? For that matter, how are
they evolving today? As pointed out by Oparin (79), it is inconceivable that a self-reproducing unit as complicated as a
nucleoprotein could suddenly arise by chance; a period of
evolution through the natural selection of organic substances
of ever-increasing degrees of complexity must intervene.
Horowitz (40) suggests a plausible scheme by which biosynthetic pathways can evolve from the successive depletion and
interconversion of related metabolites in a primitive environment, as the rich supply of organic molecules is consumed by a
burgeoning population of heterotrophs. Thus, a possible sce-
EVOLUTION OF BIOCHEMICAL PATHWAYS
A number of events initiated by carbon source starvation can
facilitate the evolution of a new catabolic pathway. Under
these circumstances, cells with gene duplication and higher
enzyme levels have a selective advantage (87, 95). In some
systems, duplicated segments are specifically subject to higher
mutation rates (93), providing ideal and expendable material
for mutations representing minor modifications of existing
genes (58). These new genes can encode modified enzymes
catalyzing reactions closely related and/or complementary to
those in existence (56). An additional consequence of starvation is the removal of feedback controls, resulting in the derepression of genes previously inhibited by the now absent metabolite. Increased rates of mutation in these derepressed
genes increase the probability of creating a new gene-enzyme
system. A number of examples exist in which derepression of a
gene has enabled an enzyme to use a new substrate. For example, altros-galactoside can be used by ␤-galactosidase after
it is derepressed (53); other examples are ␤-glycerolphosphate
via alkaline phosphatase (100), putrescine via diamine-␣-ketoglutarate transaminase (44), and D-mannitol via D-arabitol dehydrogenase (55).
An excellent example of the evolution of biochemical pathways involves the modification of two genes to serve the new
demands imposed by carbon source starvation (56, 112). Ribitol dehydrogenase, which is induced by ribitol in wild-type
* Mailing address: Division of Biological Sciences, The University of
Montana, Missoula, MT 59812. Phone: (406) 243-6676. Fax: (406)
243-4184. E-mail: [email protected].
2993
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Division of Biological Sciences, The University of Montana,
Missoula, Montana
2994
MINIREVIEW
J. BACTERIOL.
TABLE 1. Evolution of a catabolic pathwaya
Strain
b
X
X1c
X2
X3
a
b
c
Km (mM)
Generation
time (h)
Xylitol
Ribitol
[14C]xylitol uptake
(cpm)
4.1
4.1
1.7
0.9
0
290
120
130
3
3
2
2
104
104
185
685
Data taken from reference 112.
Wild-type strain with inducible ribitol dehydrogenase.
Mutant strain with constitutive ribitol dehydrogenase.
SPECIFICITY OF STARVATION-INDUCED
DEREPRESSION
Starvation for any essential nutrient activates systems that
protect the vulnerable cells from environmental damage (37,
72, 91). In addition, elaborate and specific feedback mechanisms are deployed that counteract the particular crisis created
by the absent nutrient (9, 13, 39, 54, 66, 77). For example,
inorganic phosphate (Pi) starvation derepresses the pho regulon, including a new high-affinity Pi transport system able to
cope with lower phosphate levels, and a hydrolase able to
obtain Pi from new sources (67). Nitrogen starvation derepresses the ntr regulon, including glutamine synthetase, which
has a higher affinity for NH4⫹ than the constitutive glutamate
dehydrogenase (65). Starvation for leucine specifically targets
derepression of the genes in the leu operon (111). Regulation
of amino acid biosynthetic operons by attenuation is exquisitely sensitive (over ranges of 1,000-fold) to the need for, and
the supply of, all the amino acids (18, 49, 97). Attenuation
regulation is an impressive example of the remarkable mechanisms that have evolved to ensure the conservation of precious reserves and the derepression and activation of only
those systems essential for survival under particular conditions
of starvation. As seen in Table 2, each amino acid operon
encodes in its leader sequence a series of codons for the amino
acid product of that operon. This sets the stage for a very
complex and specific mechanism to monitor the precise
amount of amino acid required relative to the amount available (49, 107). If the Leu codons are replaced with Thr codons,
regulation of the leu operon by leucine is abolished (12).
Presumably, feedback mechanisms existing today evolved in
the past to prevent unnecessary and wasteful metabolic activ-
MECHANISMS OF MUTATIONS VERSUS
MECHANISMS OF EVOLUTION
In a scientific context, the word spontaneous is meaningless.
Every event is preceded by, and dependent upon, innumerable
known and unknown prior events and circumstances. Therefore, the work background will be used when referring to the
many environmental conditions, cellular events, and repair
processes that affect mutation rates in nature. Although background mutations, such as deamination, alkylation, and depurination, occur with low frequencies, they have characteristic,
finite activation energies under physiological conditions (31,
32, 48, 60), and the molecular mechanisms by which they occur
are well established. For example, protonation of the N-3 of
cytosine results in its deamination to uracil. This process occurs 140 times more frequently in single-stranded DNA
(ssDNA) than in double-stranded DNA (dsDNA) (in which
N-3 is hydrogen bonded to N-1 of guanine), in mismatched
base pairs, and in AT-rich regions that “breathe.” These background mutations and frameshift-induced deletions and additions are DNA sequence directed in that they occur most
frequently in ssDNA and in unpaired, mispaired, and methylated bases (15, 26, 28, 31, 32, 60). Such vulnerable bases can
arise as a consequence of slippage in tandem repeats or as a
result of stem-loop DNA secondary structures that arise from
sequences containing intrastrand inverted complements. The
loops in these structures consist of unpaired bases particularly
vulnerable to mutations (other more complex structures containing vulnerable bases are also important as precursors of
mutations but will not be discussed in this minireview).
In an evolutionary context, we are not concerned with the
above molecular mechanisms by which individual types of mutation occur but with another kind of mechanism. Evolution
depends upon events that enhance mutation rates, thus increasing the supply of variants from which the fittest are selected. Therefore, the word mechanism in the present context
will refer to the circumstances affecting mutation rates. That
any DNA-destabilizing event will increase mutation rates is
TABLE 2. Leader sequences of attenuation-regulated operons
Operon
Amino acid sequence of codons in leader RNAa
his..................Met-Thr-Arg-Val-Gln-Phe-Lys-His-His-His-His-His-HisHis-Pro-Aspphe.................Met-Lys-His-Ile-Pro-Phe-Phe-Phe-Ala-Phe-Phe-Phe-ThrPhe-Prothr ..................Met-Lys-Arg-Ile-Ser-Thr-Thr-Ile-Thr-Thr-Thr-Ile-ThrIle-Thr-Thr-Glyleu..................Met-Ser-His-Ile-Val-Arg-Phe-Thr-Gly-Leu-Leu-Leu-LeuAsn-Ala-Phea
Operon-specific sequences shown in bold type.
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Enterobacter arogenes (strain X in Table 1), is unable to use
xylitol. Starvation for ribitol in the presence of xylitol results in
a mutation to strain X1, in which ribitol dehydrogenase is
constitutive and able to use xylitol, which is a poor substrate for
the enzyme but not its inducer. By repeated growth cycling on
xylitol, derivative mutants X2 and X3 are obtained with lower
Km for xylitol. The enhanced uptake of labeled xylitol in the
final mutant, X3, is due to the acquisition of a constitutively
expressed active transport system for xylitol, originating from
the modification of an inducible transport system for D-arabitol. Thus, two preexistent gene-enzyme systems evolve to initiate a new catabolic pathway in response to the stress of
imminent starvation.
In the evolution of a growth rate-limiting amino acid biosynthetic pathway, starvation, derepression, and higher mutation rates can result in a lower Km for the rate-controlling
endogenous precursor of the pathway or in the ability to use a
new and more plentiful precursor for the synthesis of that
amino acid.
ities by coordinating these activities with the presence or absence of nutrients in the environment. High mutation rates in
derepressed genes prepare cells to respond rapidly to new
challenges should the stress become more severe. As will become apparent, genetic derepression may be the only mechanism by which particular environmental conditions of stress
target specific regions of the genome for higher mutation rates
(hypermutation). Although this direct avenue for increasing
variability is probably not available to multicellular organisms
in which germ cells and somatic cells are separated, the derepression of biosynthetic pathways is essential to increased longevity in mammals subjected to caloric restriction (54), and
amino acid limitation in rats can also induce gene expression
(9).
VOL. 182, 2000
RANDOM VERSUS NONRANDOM
HYPERMUTATION
As discussed above, background mutations are sequence
directed and not random in the sense that they occur in bases
made vulnerable by virtue of their particular location within
specific DNA sequences, such as tandem repeats, or the unpaired and mispaired bases of stem-loop structures. Dobzhansky’s statement (22) enlarges upon this point: “The structure of
a gene is a distillate of its history, and the mutations that may
occur in a gene are determined by the succession of environments in which that gene and its ancestors existed since the
beginnings of life. The environment prevailing at the time
mutation takes place is only a component of the environmental
complex that determines the mutation.” The definitions of
directed and random that are appropriate in the above context
are neither relevant nor useful, however, when discussing
mechanisms of evolution. By the neo-Darwinian definition, a
mutation is random if it is unrelated to the metabolic function
of the gene and if it occurs at a rate that is undirected by
specific selective conditions of the environment. For example,
mutagenic DNA-destabilizing events associated with cell division are random, as they are dependent upon growth rate and
selective conditions of the environment only insofar as those
conditions affect the rate of cell division. However, the focus of
this minireview concerns the consequences of environmental
stress on evolution. What are the DNA-destabilizing processes
operative in stressed, nongrowing organisms forced to mutate
before they can continue to multiply? Mechanisms must have
evolved in starving cells to stimulate metabolic changes and
mutations that facilitate adaptation to new circumstances.
With the above neo-Darwinian definition of random in
mind, an impressive array of circumstances that enhance background mutation rates in response to environmental stress may
be examined with respect to whether or not they are random
(undirected). Examples of conditions that result in undirected,
genomewide hypermutation include those caused by UV irradiation, reactive oxygen species, mismatch repair-deficient mu-
2995
tator phenotypes (35, 98), horizontal gene transfer by transduction with a viral particle, and mobile genetic elements that
increase mutation rates by inserting at particular regions or at
target sequences within the genome (73, 76). Such mechanisms
are undirected because, for example, a mismatch repair deficiency will result in failure to repair a particular kind of lesion
regardless of whether or not it confers a selective advantage
upon its host.
In higher organisms, environmental conditions of stress do
not have direct access to the cells involved in reproduction, and
different mechanisms resulting in hypervariation have evolved.
For example, localized DNA rearrangements and shuffling
produce extensive beneficial variation (82, 96), and hypervariable sequences provide continual changes in the composition
of venoms produced by snakes (29) or snails (78) to overcome
resistance developed by their predators or prey. These mechanisms are also random. The threat of predators does not
result in hypermutation; there is no evidence that the circumstances selecting such hypermutable genes bear any metabolic
relationship to the mechanisms by which they originally arose.
A gene may be hypermutable because it contains a hot spot
due to a particular DNA sequence, and if a high mutation rate
is advantageous to its host, that gene will be selected during
evolution. However, its hypermutability per se is undirected,
since it is unrelated to those selective conditions and to the
function of the gene. These random mechanisms resulting in
hypermutation are in essence serendipitous relationships; in
contrast, hypermutation resulting from derepression is localized as a direct consequence of a specific response to environmental challenge.
TWO MECHANISMS BY WHICH TRANSCRIPTION CAN
INCREASE MUTATION RATES
Transcription exposes ssDNA. The most common base substitution events in the spectra of background mutations in
E. coli and mammalian cells are G 䡠 C-to-A 䡠 T transitions. Fix
and Glickman (28) observe that 77% of these mutations originate on the nontranscribed strand in E. coli mutants unable to
repair deaminated cytosines. This suggests that the unprotected single strand in the transcription “bubble” is significantly more vulnerable to mutations than the transcribed strand,
which is protected as a DNA-RNA hybrid (Fig. 1A). The frequency of UV-induced lesions in the lacI gene is also higher in
the nontranscribed strand than in the transcribed strand (46).
In fact, cytosines deaminate to uracils in ssDNA at more than
100 times the rate in dsDNA (31, 32, 60). The relative mutability of the nontranscribed strand is also seen in a plasmid
system in which a fourfold increase in the frequency of transitions occurs selectively in the nontranscribed strand when
transcription is induced (4). Transcription may therefore be a
prerequisite for many C-to-T transition mutations, since other
mechanisms resulting in the transient generation of singlestranded sequences, such as replication or breathing (102) do
not lead to asymmetry in the two strands. Apparently, the observed strand bias cannot be explained by transcription-coupled repair (36), since base mismatches are poor substrates for
this kind of repair, and the same strand bias is observed when
the host is deficient in repairing U 䡠 G and T 䡠 G mismatches
(4). Thus, transcription may be implicated as a major cause of
background transition mutations in nature.
Transcriptional activation as a mechanism for increasing
mutation rates was first proposed in 1971, by Brock (8) and
Herman and Dworkin (38). Their work demonstrates that
recA-independent lac reversion rates of frameshift and point
mutations are higher when transcription is induced by isopro-
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axiomatic. In growing cells, such events include polymerase
and proofreading errors during DNA replication, as well as
recombination, transcription, and repair. DNA transcription
and repressor binding affect the rate of deletions in Escherichia
coli plasmids (101). The availability of ssDNA (leading- and
lagging-strand DNA templates) facilitates the slippage of tandem repeats and the formation of stem-loop structures (89). In
nongrowing cells, however, the DNA-destabilizing events of
replication are probably not primary causes of mutations.
Within an hour following starvation, bacterial cells undergo
major metabolic transitions (the stringent response [13]) in
which genes required for cell division are repressed while a
number of other genes (depending upon the starvation regimen) are derepressed. During this transition from exponential
growth to stationary phase, events related to gene activation
parallel a sharp increase in supercoiling, suggesting that transcriptional activation may drive supercoiling and the resulting
DNA secondary structures that are precursors of mutations
(discussed below). Among the known DNA-destabilizing
events, only transcription can be selectively activated (7), either by induction or derepression. Derepression of the leu
operon in E. coli is specifically correlated with an increased
rate of leuB mRNA turnover (62; J. M. Reimers, A. Longacre,
and B. E. Wright, Conf. DNA Repair Mutag., abstr. B29, p. 80,
1999) and an increased reversion rate of the leuB mutant gene;
this mutation is located at the site of a predicted stem-loop
structure (111).
MINIREVIEW
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MINIREVIEW
pyl-␤-D-thiogalactopyranoside (IPTG), and that the effect is
specific. More recently, specifically induced, transcription-enhanced mutations have also been shown for a lys frameshift
mutation in Saccharomyces cerevisiae (16, 74). Starvation-induced stringent response mutations in E. coli (62, 109–111)
and Bacillus subtilis (90) occur as a result of transcriptional
activation triggered by gene derepression, not induction. In
this system, mutations arise during the transition between
growth and stationary phase and they are recA independent,
similar to the lac reversions mentioned above. This distinguishes them from prolonged stress-induced adaptive mutations (11) and from DNA damage-induced SOS mutagenesis
(104), both of which require recA (and will not be discussed in
this minireview). It is noteworthy that the experiments described above on the effects of artificially induced transcription
on mutation rates in growing cells are all examples of specifically directed mutations. However, none of the researchers
come to that conclusion or challenge the assumptions and
implications inherent in the experiments of Luria and Delbruck (63), which reinforce neo-Darwinism. This situation may
be due to the dominance of current dogma and to the assumption that mechanisms operative during growth cannot also be
critical during evolution under conditions of environmental
stress. In fact, the limited evidence now available suggests that
only growing cells, or cells in transition between growth and
stationary phase, have the metabolic potential required for
specific, transcription-induced mutations in response to environmental challenge. Thus, IPTG induction enhances lac reversion rates in growing cells (38) but not in cells subjected to
prolonged stress (17). Transcriptional activation is the mechanism for enhancing mutation rates both in the artificially
induced systems and in stringent response mutations (90, 111;
J. M. Reimers, A. Longacre, and B. E. Wright, Conf. DNA
Repair Mutag., 1999). However, only the latter are relevant to
evolution, since they occur naturally as a result of starvationinduced derepression. Mutations that most benefit organisms
and accelerate evolution may occur as an immediate response
to imminent starvation, when cells still have the metabolic
resources to respond specifically to the particular conditions of
stress at hand.
Transcription drives localized supercoiling. Chromosomal
DNA from bacterial cells is negatively supercoiled. The level of
global negative supercoiling in E. coli cells is maintained within
a physiologically acceptable range by two opposing enzyme
activities: DNA gyrase, which introduces negative supercoils,
and topoisomerase I, which relaxes them. Investigations with
plasmids grown in E. coli (59, 81) demonstrate the presence of
stem-loop structures in naturally occurring supercoiled circular
DNA molecules (Fig. 1B). Analyses with single strand-specific
nuclease show that DNA molecules with high superhelical
densities are selectively cleaved, in contrast to their linearized
counterparts with which they are in dynamic equilibrium in
vivo. The sequence surrounding the area of cleavage reveals
inverted complementary sequences that hydrogen bond to become the stem separated by noncomplementary bases that
become the single-stranded loop and the substrate for nuclease
cleavage. Such complex structures form preferentially in easily
denatured AT-rich stretches of DNA and occur about 10,000
times more frequently than expected by chance (30, 59), suggesting their selection during evolution. Data indicate that
stem-loop-based recombination may have evolved in the early
“RNA world” (94) and that the potential to generate stemloops was later conserved, for example, in hypervariable snake
venom genes under strong selection to keep one step ahead of
both predators and prey (29).
A number of variables, such as temperature, anaerobiosis,
osmolarity, and nutritional shifts, affect DNA supercoiling (1,
19, 47, 85, 86). Some environmental perturbations affect plasmid systems and chromosomes in a similar manner, while some
apparently do not (25). Transcription both responds to and
promotes changes in supercoiling. The optimal level of supercoiling for gene expression varies for different genes, and su-
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FIG. 1. (A) Exposure of the nontranscribed strand during transcription; (B)
effect of transcription on supercoiling; (C) a typical stem-loop structure containing unpaired and mispaired bases; (D) mutation 1, a C-to-T transition in the loop.
J. BACTERIOL.
VOL. 182, 2000
percoiling-induced conformational changes may be required
for structural changes in regulatory complexes and for recognition by RNA polymerase (RNAP) (85). Transcription has a
profound effect on supercoiling, because RNAP distorts and
destabilizes dsDNA. As indicated in the twin-domain model
(Fig. 1B) of Liu and Wang (61), negative supercoiling is generated behind, and positive supercoiling in front of, the advancing RNAP transcription complex. Many investigations
provide evidence demonstrating that transcription drives supercoiling in vivo (1, 19, 20, 27, 86) and that the wave generated can be as long as 800 bp (47). Negative supercoiling
induces and stabilizes a transition from the right-handed Bform to the left-handed Z-form of DNA (42); a chemical assay
detecting these distortions reveals that transcription-induced
supercoiling is highly localized (47, 86). During the induction
of transcription, supercoils are found inside each transcribed
region, as well as upstream and downstream of each individual
RNAP complex. Transcription from a strong promoter leads to
greater negative supercoiling than transcription from a weak
one (27). A major role of DNA topoisomerase I is now considered to be the relaxation of local negative supercoiling during transcription, thus preventing unacceptably high levels of
supercoiling and associated R-loops that form when nascent
RNA moves behind the advancing RNAP to bond with its
original template DNA (69, 70, 105). The bulk of plasmid
DNA does not exhibit stem-loop conformations during logarithmic growth. However, supercoiling may play a particularly
important role in stressed cells, in which a disruption can occur
between transcription and translation, thus promoting both
R-loop formation and supercoiling (68, 69). The inhibition of
protein synthesis by chloramphenicol, which uncouples transcription and translation, induces stem-loop formation in the
overwhelming majority of DNA molecules (19).
When E. coli is grown with limiting levels of glucose (Fig. 2),
a burst in supercoiling occurs precisely at the moment of glucose depletion, as the cells cease logarithmic growth and enter
stationary phase (1). This is also the moment at which a sharp
increase occurs in the concentration of cyclic AMP (cAMP)
(10), ppGpp (23, 51), ␴S (50), and about 30 new proteins,
including ␤-galactosidase (34, 37). The increase in negative
supercoiling under these circumstances can be due to the increase in transcription known to occur as a result of derepres-
2997
sion in response to starvation for any essential nutrient. The
two “alarmones,” cAMP and ppGpp, activate transcription in
derepressed genes in a number of systems. Both are required
for the synthesis of enzymes that catabolize alternative carbon
sources, such as ␤-galactosidase (83, 84, 91). The alarmone
ppGpp activates the synthesis of ␴S (33), which in turn governs
the expression of a number of stationary-phase genes involved
in the starvation-mediated resistance to osmotic, oxidative, and
heat damage (37, 72). Under circumstances in which a group of
related genes become activated, such as those dependent upon
␴S, the topological changes in DNA could provide a mechanism
by which transcriptional activation in one gene may influence
adjacent genes (47, 85). Changes in stem-loop formation and
superhelicity similar to that caused by glucose starvation (Fig.
2) are also observed immediately following amino acid starvation or the inhibition of protein synthesis (19). Within 30 min
following treatment with chloramphenicol or valine (which creates an isoleucine deficit and ppGpp accumulation in E. coli),
stem-loop formation is evident. Isoleucine starvation results in
the formation of stem-loops in a much smaller number of
DNA molecules than are affected by chloramphenicol inhibition, consistent with the selective derepression of relatively few
genes in the absence of a single amino acid.
In response to starvation for any essential metabolite, the
immediate problem is addressed specifically (e.g., derepression
of a higher-affinity transport system for that metabolite), coupled with a general increase in stress resistance. Starvation
results in derepression, and transcription drives localized supercoiling; the formation of stem-loop structures at regions of
high superhelicity results in localized hypermutation (Fig. 1).
Although energetic considerations do not favor the creation of
complex structures in metabolically inactive dsDNA, transcription clearly accelerates supercoiling and transitions to secondary DNA structures (1, 19, 20, 27, 47, 59, 69, 70, 81, 86).
SECONDARY DNA STRUCTURES: ARE THEY
PRECURSORS TO MUTATIONS?
Almost 40 years ago, Benzer (5) demonstrated that the mutability of specific sites in the genome varied by orders of
magnitude, suggesting that these differences in mutation rate
might reflect particular characteristics of the DNA sequence
associated with hot spots. In fact, mutable sites are frequently
the consequence of their location within DNA secondary structures. Simple stem-loops arise from ssDNA sequences containing two segments that are inverted complements, usually about
10 to 15 bases long, separated by 5 to 10 noncomplementary
bases that become the loop at the end of the stem formed by
hydrogen bonding of the two complementary segments (Fig.
1C). These structures are called hairpins if the loop is very
small and cruciforms if they form opposite one another in each
DNA strand. Perfect complementarity (a palindrome) is rare;
the more common quasipalindromes or stem-loops contain
bases that are left unpaired or mispaired and therefore vulnerable to deamination (mutations 1 and 2 in Fig. 1C), deletion
(mutations 3 and 4), replacement (mutations 5 and 6), or
complementation by the insertion of a new base to the structure (mutation 7). The stem-loop is in dynamic equilibrium
with linearized DNA, and changes such as those indicated in
Fig. 1C only become fixed as mutations in the course of further
metabolic events such as repair or replication (Fig. 1D). For
example, the entire structure depicted in Fig. 1C will be excluded and deleted by virtue of new DNA synthesis across the
base of its stem. However, if this structure returns to its linear
form prior to new DNA synthesis, the potential changes indicated above can be immortalized due to synthesis templated by
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FIG. 2. Depletion of limiting levels of glucose by E. coli immediately triggers
several metabolic events. Depletion of a limiting amino acid has similar consequences. Curve A represents growth (optical density). Curve B represents the
concentration of cAMP, ppGpp, ␴S, ␤-galactosidase, 30 other proteins, and
supercoils.
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MINIREVIEW
FIG. 3. An algorithm for evolution.
that the sequence-dependent secondary structures created and
stabilized by supercoiling are precursors to mutations.
CONCLUSIONS
Many scientists may share Dobzhansky’s intuitive conviction
that the marvelous intricacies of living organisms could not
have arisen by the selection of truly random mutations. This
minireview suggests that sensitive, directed feedback mechanisms initiated by different kinds of stress might facilitate and
accelerate the adaptation of organisms to new environments.
The specificity in the series of events summarized by Fig. 3
resides entirely in the first step, which is meant to suggest a
pattern of derepression elicited by a corresponding pattern of
adverse conditions. Microorganisms in nature must be confronted simultaneously by a complex set of problems, for example, the threat of oxidative or osmotic damage together with
suboptimum concentrations of many essential nutrients. Transcriptional activation of genes derepressed to various degrees
would expose the nontranscribed strands to mutations and
stimulate localized supercoiling. Vulnerable bases in the complex DNA structures resulting from supercoiled DNA will also
contribute to localized hypermutation in the genes activated to
cope with the stresses that initiate the above series of events.
A multitude of random mechanisms result in hypermutation
under conditions of environmental stress and clearly contribute to the variability essential to evolution. However, since
most mutations are deleterious, random mechanisms that increase mutation rates also result in genomewide DNA damage.
Among microorganisms, from phage to fungi, the overall mutation rate per genome is remarkably constant (within 2.5fold), presumably reflecting an obligatory, delicate balance between the need for variation and the need to avoid general
genetic damage (24, 45, 57). Thus, mutator strains are not
selected in nature but remain at 1 to 2% of the population (35,
52); under certain adverse conditions, they flourish for short
periods but are then selected against, apparently because of
widespread deleterious effects intrinsic to genomewide hyper-
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the altered sequence. An example of this process is indicated in
Fig. 1D, in which a C in the loop is deaminated to uracil, which
codes for A, which then codes for T during DNA synthesis,
resulting in a C-to-T transition.
What is the probability that these structures actually exist in
vivo and constitute precursors of background mutations in
nature? One kind of evidence for their existence is the striking
correlation of deletion end points with tandem repeats and the
ends of potential secondary structures. The leuB, argH, and
pyrD mutations in E. coli all occur at the end of predicted
stem-loops (111). The lacI gene has frequently been used as a
model system for investigating these correlations by comparing
its nucleotide sequence (26) to those of various mutant strains.
Among the sequenced deletion mutations in lacI, 7 occur in
tandem repeats, 6 consist of deleted stem-loop structures, and
4 of these 13 mutations share both characteristics. Although
three remaining mutations fail to coincide exactly with predicted vulnerable sites, they may be explained by different
DNA secondary structural intermediates including interstrand
misalignments (26, 88, 92). Todd and Glickman (99) analyzed
102 amber and 71 ochre mutants in the lacI gene, correlating
mutation hot spots with the locations of predicted unpaired
sites, many of which are located in stem-loop structures. Mutational hot spots are highly localized, and 50% of the nonsense mutations arose in a segment comprising only 6% of the
DNA sequence analyzed. Each hot spot is found to be located
at an unpaired site within potential secondary structures.
Clearly, these correlations depict causal relationships.
The mechanism of frameshift mutagenesis has also been
examined in vitro during DNA polymerization (80). In this
system, sequence misalignments result from intrastrand complementary pairings between two segments within a single
newly synthesized strand, as well as from interstrand pairings
between a segment in the new strand and a complementary
sequence in the template strand. When these misaligned segments are used as templates for DNA synthesis, mutant sequences are produced. Polymerase pausing, or the local rate of
DNA polymerization, was also measured and correlated with
the misalignments, since pausing is sequence specific as well
(43). Pausing serves to increase the time of exposure of mutable bases and is known to promote mutagenesis (2, 3). The
correlation between pausing, positions of frameshift misalignments, and subsequent deletions can account for 97% of the
mutations observed. Moreover, the most common mutations
coincide precisely with the strongest pause sites, and the termini of pause sites correlate with the sequence at which the
deletions begin.
Thus, in vivo and in vitro investigations strongly implicate
the existence of DNA secondary structures as mutagenic substrates and/or as structural precursors to mutagenic substrates
that give rise to mutations that are immortalized during new
DNA synthesis or repair (Fig. 1D). As discussed earlier, mutations occur preferentially in ssDNA and in unpaired and
mispaired bases (28, 31, 32, 48, 60). Other kinds of evidence
also support a role for secondary structures as precursors of
mutations. Many insertion mutations (Fig. 1C) can best be
explained if the other strand of a predicted transient stem were
used as a template for DNA synthesis prior to replication. The
fact that mutations are grouped closely together much more
frequently than could occur by chance implicates a single initiating event (structure). As the researchers of the above investigations point out, other variables undoubtedly contribute
to and modify the correlation observed between DNA sequences, misaligned structures, polymerase pausing, and mutations. Nevertheless, these investigations are but a small fraction of an enormous literature providing compelling evidence
J. BACTERIOL.
VOL. 182, 2000
MINIREVIEW
ACKNOWLEDGMENTS
I am indebted to my colleagues in the Division of Biological Sciences
and to Lynn Ripley and Karl Drlica for suggestions and valuable
criticisms of this manuscript.
This work was supported in part by the Eppley Foundation, PHS
grant GM/OD54279, a MONTS grant from the NSF EPSCoR program, and by the University of Montana.
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