© 2001 Nature Publishing Group http://structbio.nature.com
history
How jumping genes were discovered
© 2001 Nature Publishing Group http://structbio.nature.com
Nina Fedoroff
The discovery of transposition can be
dated quite precisely from comments
penned in Barbara McClintock’s hand on
an unpublished manuscript from January
of 1949. Seeking to explain the behavior of
a chromosome breaking site she named
the Dissociation (Ds) locus, McClintock
says: “At the time, I did not know that Ds
could change its location. Realization of
this did not enter my consciousness until
late this spring, following the harvest of
the greenhouse crop”1. The spring was
that of 1948 and the greenhouse crop was
grown in the winter 1947–1948.
While it was McClintock’s unique
insight that certain genetic elements move
around or transpose, the kinds of unstable
mutations now known to be caused by
transposons had been studied for many
decades. The maize geneticist Emerson
(1914) worked with a type of corn whose
kernels showed stripes of dark red pigmentation2. Although earlier geneticists
had failed, Emerson was able to understand the behavior of such an unstable
mutation within the Mendelian paradigm.
He suggested that it was the result of a
transient association of an inhibiting factor with a gene required for pigmentation
and that this interaction gave rise to the
unpigmented phenotype. He further postulated that the factor could lose its power
to inhibit the gene during development,
thus explaining the origin of pigmented
sectors and fully pigmented kernels. But
he could not imagine what this inhibitor
could be. Later Rhoades (1941) made the
important observation that a certain stable null mutation at the maize A locus
became unstable when combined with a
locus he designated Dotted3.
McClintock’s discovery of transposition
had its origins in her studies on broken
chromosomes in corn1. She observed a
pattern of breakage on chromosome number 9 that resulted in the repeated loss of
the same chromosome fragment during
development, and she named the breakage
site the Ds locus. The regular breakage or
dissociation of the chromosome at this
site contrasted with the more familiar pattern of random chromosome breakage
resulting from the formation of chromosomes containing two centromeres. These
dicentric chromosomes are unstable and
break when the two centromeres are
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pulled to opposite poles during mitosis.
Analyzing the Ds locus further,
McClintock soon identified a second locus
that was required to activate chromosome
breakage, and she designated this the
Activator (Ac) locus.
She also noticed that the progeny of
plants with broken chromosomes had an
unusually high frequency of variegation.
Some showed a change from a mutant to a
normal appearance, much like the mutations studied by Emerson and Rhoades.
She also noticed something else. Every
now and then ‘twin sectors’ would arise
where one part of the kernel or plant
(depending on where the affected gene is
expressed) showed increased while the
other showed decreased rates of mutation.
Importantly, McClintock noted that the
frequency and timing of variegation and
chromosome breakage were linked.
By the time she wrote her summary for
the 1947–1948 Carnegie Institution of
Washington Yearbook, McClintock was
able to say: “It is now known that the Ds
locus may change its position in the chromosome….”1. This statement was followed by a summary of the genetic
experiments that led her to this startling
conclusion. The clue she followed to the
discovery of transposition was a change in
the pattern of chromosome 9 marker loss
following chromosome breakage at the Ds
locus caused by Ac. After an intensive
genetic analysis (the detailed description
of which runs to 50 pages and more than
40 tables and diagrams), McClintock concluded that the pattern change resulted
from the movement of Ds to a new location distal to the original one on the same
chromosome, accompanied by a large
inverted duplication of the intervening
chromosome segment.
The relationship between Ds and the
new unstable mutations emerged when
McClintock studied a new allele of the C
locus (a gene required for the expression
of genes in the anthocyanin pathway that
gives plants their purple, pink, red and
blue colors) on chromosome 9. Ac caused
both instability of the mutation (variegation) and chromosome breakage at this
locus. But when the mutation reverted,
chromosome breakage no longer occurred
at the locus. The last piece of the puzzle
had fallen in place and McClintock was
able to explain the basis of variegation that
had by then been under genetic scrutiny
for almost half a century. Unstable mutations of the type analyzed by both
Emerson and Rhoades were caused by
transposon insertions that disrupted
genes. The transposon excised frequently
during development, restoring gene function. The twin sectors McClintock reported were the result of transpositionassociated increases and decreases in
transposon copy number.
Today we know that Ac is a 4.5 kb transposon that encodes a single transposase
protein4. Ds elements are often, but not
always, internally deleted, transpositionincompetent derivatives of Ac. Ds elements
transpose when supplied with Ac-encoded
transposase and chromosome breakage is
not a general property of Ds transposons.
It turns out that McClintock’s chromosome-breaking Ds is a composite element,
with one internally deleted 2 kb transposon inserted in inverted order precisely
into the middle of a second copy. This
composite transposon contains both termini in both orientations, resulting in the
cleavage of both, rather than just one sister
chromatid during transposition. This is
often followed by joining of the broken
chromatid ends, creating a dicentric and
an acentric chromosome fragment.
Today we also know that transposons
constitute a large fraction of the DNA in
some species of plants and animals, among
them mice, humans, corn and wheat. It is
paradoxical that the discovery of transposable elements lagged so far behind the
discovery of the basic laws of genetic transmission. And it is equally curious that even
when they were discovered, acceptance of
their generality and recognition of their
ubiquity came so slowly. One explanation
may be that transposons are all but invisible to the geneticist. Insertion mutations in
genes are relatively infrequent (for example, one in several hundred mutations in
humans) and chromosomes containing
many hundreds of thousands of transposable elements are as stable as chromosomes
containing few. Although we now understand much about the mechanics of transposition, we still know rather little about
how transposition is controlled both developmentally and temporally. Finally, why is
it that genomes are not constantly scram-
nature structural biology • volume 8 number 4 • april 2001
© 2001 Nature Publishing Group http://structbio.nature.com
history
© 2001 Nature Publishing Group http://structbio.nature.com
bled by recombination between similar or
identical sequences at different places in the
genome? Clearly, transposons must be
sequestered from the recombination
machinery in some way that is at this point
very poorly understood.
Nina Fedoroff is the Director of the Life
Sciences Consortium and Biotechnology
Institute and is in the Biology Department,
Pennsylvania State University, University
Park, Pennsylvania, 16802, USA. email:
[email protected]
1. McClintock, B. The discovery and characterization
of transposable elements. The collected papers of
Barbara McClintock. (Garland, New York;1987).
2. Emerson, R. A. Am. Nat. 48, 87–115 (1914).
3. Rhoades, M. M. Cold Spring Harbor Symp. Quant.
Biol. 9, 138–144 (1941).
4. Fedoroff, N. In Mobile DNA (eds Howe, M. & Berg,
D.) 375–411 (American Society for Microbiology,
Washington; 1989).
picture story
The separator
Dynamin is essential for receptor-mediated endocytosis, a mechanism by which a
cell obtains nutrients (such as iron or cholesterol) from its environment. In this
process, the membrane, which contains
receptors and their bound ligands, folds
inward to form a ‘pit’; the pit then closes
up to form a vesicle that breaks off from
the membrane.
Dynamin oligomerizes around the neck
of the pit and is involved in cutting off the
vesicle from the membrane, but its exact
role in the process is under considerable
debate. Dynamin is a GTPase and, in principle, could provide the mechanical force
— derived from GTP hydrolysis — to
pinch off the vesicle from the membrane.
It is also possible that dynamin functions
as a classic G-protein — that is, a molecular switch that senses the state of the
bound nucleotide to regulate downstream
scission factor(s). In this case, GTP binding, but not GTP hydrolysis, would be sufficient for endocytosis.
To differentiate between these two
hypotheses, Marks et al. (Nature, 415,
231–235; 2001) mutated residues that
may be important for the GTPase activity
of dynamin. One such mutant, T65A, has
an affinity for GTP similar to that of wild
type but lacks the hydrolytic activity.
Importantly, this mutant does not support endocytosis in vivo of a model substrate, transferrin (compare the cells
(colored red) expressing wild type
dynamin (top left) to those expressing
T65A dynamin (bottom left); transferrin
is highlighted green), indicating that
GTP binding alone is insufficient for
endocytosis.
Transferrin endocytosis
GTPγS
25% GTP
hydrolysis
WT
WT
WT
T65A
K142A
K142A
adapted from Marks et al.
Another mutant, K142A, is not defective
in GTP binding or hydrolysis but still does
not support endocytosis. When bound to
GTPγS, purified wild type dynamin assembles in vitro into a tight helix around lipid
nanotubes (top middle). After GTP hydrolysis a concerted conformational change
occurs that alters the pitch of the dynamin
helix (top right). The K142A mutant can
assemble into a helix similar to that of wild
nature structural biology • volume 8 number 4 • april 2001
type in the presence of GTPγS (bottom
middle); however, the conformational
change after GTP hydrolysis is seriously
compromised (bottom right). This observation suggests that a concerted conformational change of dynamin is also required
for endocytosis. Taken together, these findings support the hypothesis that dynamin
can act as a force generator to separate vesicles from the membrane. Hwa-ping Feng
301