Biochemical Society Transactions (2002) Volume 30, part 4
This work was supported by grants from the Biotechnology and
Biological Sciences Research Council, U.K.
9
10
References
I
2
3
4
5
6
7
8
II
Griffiths, W. T. ( 1978) Biochem. J. 174, 68 1-692
Lebedev, N. and Timko, M. P. ( I 998) Photosynth. Res. 58,
5-23
Aubert, C., Vos, M. H., Mathis, P., Eker, A. P. and Brettel, K.
(2000) Nature (London) 405, 586-590
Baker, M. E. ( I 994) Biochem. J. 300, 605-607
Townley, H. E., Sessions, R. B., Clarke, A. R., Daffom, T. R.
and Grifiths, W. T. (2001) Proteins 44,329-335
Persson, B., Krook, M. and Jomvall, H. ( I 99 I ) Eur. J,
Biochem. 200, 537-543
Bohren, K. M., Grimshaw, C. E., Lai, C. J., Hamson, D. H.,
Ringe, D., Petsko, G. A. and Gabbay, K. H. ( 1994)
Biochemistry 33, 202 1-2032
Varughese, K. I., Xuong. N. H., Kiefer, P. M., Matthews,
D. A. and Whiteley, J, M. ( I 994) Proc. Natl. Acad.
Sci. U.S.A.9 I,5582-5586
12
13
14
15
16
17
Ensor, C. M. and Tai, H. H. ( I 99 I ) Biochem. Biophys.
Res. Cornmun. 176, 840-845
Albalat, R., Gonzalez, D. and Atrian, S. ( I 992) FEBS Lett
308,235-239
Chen, Z.,Jiang,J. C., Lin, Z.
G., Lee, W. R., Baker, M. E. and
Chang, S. H. (1993) Biochemistry 32, 3342-3346
Cols, N.,Marfany, G., Atrian, S. and GonzalezDuarte, R.
( 1993) FEBS Lett. 3 19, 90-94
Obeid, J. and White, P. C. ( 1992) Biochem. Biophys.
Res. Cornmun. 188, 222-227
Kiefer, P. M., Varughese, K. I., Su, Y., Xuong, N. H., Chang,
C. F., Gupta, P., Bray, T. and Whiteley. J. M. (1996) J, Biol.
Chem. 27 I , 3437-3444
Wilks, H. M. and Tirnko, M. P. ( I 995) Proc. Natl. Acad.
Sci. U.S.A. 92, 724-728
Lebedev, N., Karginova, O., Mclvor, W. and Timko, M. P.
(200 I ) Biochemistry 40, 12562- I2574
Heyes, D. J., Martin, G. E. M., Reid, R. J., Hunter, C. N. and
Wilks, H. M. (2000) FEBS Lett 483,47-5 I
Received I I March 2002
Making light of it: the role of plant haem oxygenases in phytochrome
chromophore synthesis
M. J. Terry"', P.J. Linley" and T. Kohchit
*School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO I 6 7PX, U.K.,
and tGraduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma,
Nara 630-0 I0 I , japan
-
Abstract
T h e haem oxygenase (HO) enzyme catalyses the
oxidation of haem to biliverdin IXa, CO and Fe2+,
and performs a wide variety of roles in Nature,
including degradation of haem from haemoglobin,
iron acquisition and phycobilin biosynthesis. In
plants, H O s are required for the synthesis of the
chromophore of the phytochrome family of photoreceptors. There are four H O genes in the Arabidopsis genome. Analysis of a mutant deficient in
H 0 1 (the hyl mutant) has demonstrated that this
plastid-localized protein is the major H O in the
phytochrome chromophore synthesis pathway.
H 0 2 may also have a minor role in this pathway,
but our understanding of the divergent roles of
this small gene family is still far from complete.
Key words: biliverdin, chloroplasts, light and plastid signalling,
photoreceptor, tetrapyrroles.
Abbreviations used: BV, biliverdin; EST, expressed sequence tag;
HO, haern oxygenase protein; HO, haem oxygenase gene; POB,
phytochromobilin.
'To whom correspondence should be addressed (e-mail
[email protected]).
0 2002 Biochemical Society
604
Haem oxygenase (HO) a universal
enzyme for haem degradation
HO was originally identified as a key enzyme
in the degradation of haem through its role in
bilirubin production in rat liver [l], and is still the
only enzyme known that can catalyse haem degradation. As shown in Scheme 1, H O oxidizes haem
at the a-meso position to give biliverdin (BV) I X a ,
Fe2+ and CO in a reaction requiring molecular
oxygen and electrons from N A D P H . T h e mechanism of this reaction has been described in detail
recently, and proceeds through a number of
relatively stable intermediates, including a-meso
hydroxyhaem and verdohaem [2]. As discussed
below, additional proteins are required to utilize
N A D P H , and the type of protein used is speciesdependent : mammalian H O s require cytochrome
P450 reductase [2], while bacterial HOs have been
shown to use ferredoxin [3,4] or putidaredoxin
[5] together with an accompanying NAD(P)Hdependent reductase.
While haem degradation is a common feature
of all HOs, this enzyme actually has a great variety of roles in Nature, due in part to the diversity
Tetrapyrroles: Their Life, Birth and Death
Another example of a role in iron metabolism
comes from the red alga Rhodella violacea, in
which a HO gene is transcriptionally activated by
iron limitation [12]. Finally, high levels of HO
expression in the mammalian nervous system,
together with the identification of physiological
responses to CO, have led to the proposal that
H O s may specifically mediate the release of C O in
neural cells, where it is thought to have a signalling
role [13].
of the reaction products. In mammals, BV I X a
is reduced to bilirubin, through the action of BV
reductase, which is then conjugated to glucuronic
acid prior to excretion [2]. In other organisms,
however, BV IXa is used for a whole range of
purposes. In cyanobacteria and algae it is the
precursor of phycobilins used for light-harvesting
during photosynthesis [6]. In plants, as we shall
see below, BV IXa is reduced to phytochromobilin
(PQB), the chromophore of the phytochrome
family of photoreceptors [7]. In reptiles, fish,
insects and the eggshells of birds, BV IXa is used
directly for pigmentation, and it has even been
shown to function as a signalling molecule during
dorsal development in Xenopus laevis embryos [S].
Even in mammals, excretion is not necessarily
the end of the story. Both BV IXa and bilirubin
have been shown to have strong antioxidant
properties in vitro [9], and this has led to speculation that HO may have a protective role against
tissue injury [lo]. Similarly, the release of iron and
subsequent induction of iron-sequestering proteins such as ferritin has also been proposed as an
important function of HOs in counteracting oxidative stress in mammalian cells [l 11. Certainly,
iron acquisition seems to be the raison d’gtre of
some bacterial HOs, such as in the pathogenic
bacterium Corynebacterium diphtheriae [S].
HOs in plants
T h e first direct evidence for the presence of H O in
photosynthetic organisms came from feeding experiments in the red alga Cyanidium caldarium
[14]. These demonstrated that the phycobiliprotein chromophore, phycocyanobilin, was synthesized from haem in a manner identical to that used
in mammalian systems. Subsequently, this system
was utilized to first measure [15], and then partially
purify [16], an algal HO. In higher plants, the
similarity of the phytochrome chromophore, PQB,
to phycobilins (such as phycocyanobilin) and the
demonstration that BV IXa was the precursor of
PQB led to the proposal that a similar pathway
might be utilized for PQB synthesis [17]. Evidence
that exogenous haem could support PQB synthesis
in isolated etioplasts [7] and that the ferrochelatase
inhibitor N-methylmesoporphyrin I X could inhibit phytochrome chromophore synthesis in the
pea [18] further supported this idea.
T h e question of the role of H O in phytochrome synthesis was finally resolved through the
analysis of mutants unable to synthesize PQB [ 191.
Biochemical analyses of the phytochrome chromophore deficient 1 ( p c d l ) mutant of pea [20] and
the yellow-green-2 ( y g - 2 ) mutant of tomato [21]
demonstrated that both of these phytochromedeficient mutants lacked HO activity. T h e positional cloning of an equivalent mutant of Arabidopsis, h y l , demonstrated that the affected gene
showed sequence similarity with mammalian and
cyanobacterial HOs [22,23] and that the gene
product possessed HO activity in vitro [22]. This
gene was named H 0 1 , and encodes a 282-aminoacid protein with a predicted molecular mass of
32.6 kDa. This included a 55-amino-acid chloroplast transit peptide, predicting a mature protein
of 26.6 kDa following cleavage of the transit
peptide. A chloroplast location for HO1 was
confirmed using a green fluorescent protein reporter, and immunoblot studies demonstrated
that HO1 was found predominantly in the stroma
Scheme I
The HO reaction
Substrates and products are not shown stoichiometrically
Heme
I
H02C
CO2H
NADPH102
4
k
H
co/Fa**
H
H
Biliverdin IXa
[221.
605
0 2002 Biochemical Society
Biochemical Society Transactions (2002) Volume 30, part 4
H 0 1 genes have now been identified from
a number of plant species, including rice 1261,
tomato 1251 and pea (P. J. Linley, M. Landsberger, T. Kohchi and M. J. Terry, unpublished work), and are present in numerous
E S T (expressed sequence tag) collections. Given
the plastid localization of H 0 1 , one of the surprising features of the phylogenetic analysis shown
in Figure 1 is that higher plant sequences are
equally divergent from mammalian and cyanobacterial or algal (including those present on the
algal plastid genome [12]) sequences. Despite this,
T h e Arabidopsis genome-sequencing programme has now revealed that HOs comprise a
small gene family, with four members in total
124,251. As shown in Figure 1, these genes fall into
two distinct classes : HOI-like genes (including
H 0 3 and H 0 4 of Arabidopsis) and H 0 2 genes
[25]. Interestingly, while all plant HO1-like
genes contain the conserved histidine that functions as the proximal haem ligand (His-25 in mammalian H 0 1 [2]), in the H 0 2 genes this is replaced
by an arginine. T h e functional implications of
such a substitution are not yet clear.
Figure I
Phylogenetic tree showing animal, plant and bacterial HOs
H O protein sequences (34 sequences from 24 species) were aligned using the ClustalW algorithm [45]. The bar is equivalent to 0.I amino
acid substitutions per position ( i e a 10% difference in amino acid sequence identity), and for the purposes of the phylogenetic alignment
the N-terminal transit peptides of plant sequences and the Cterminal membrane-anchor domains of animal sequences were removed. The
accession numben of the sequences used were as follows. Plant HOs: pine (Pt H O I , AF320030), Medicogo truncotulo (Mt H O I , ESTs
AW98 I0 I 7 and AL38 I 336), pea (Ps H O I , AF276228), soybean (Gm H O I , AF320024; Grn H 0 3 , AF320025), Arobidopsis tholrono (At H O I ,
AF I 32475 ; A t H02, AF I 32477; A t H 0 3 , AF320022; A t H04, AF320023), rye (Sc H O I ; composite sequence from ESTs BE586278 and
BE586940), rice (0s H O I , C28969), sorghum (Sb HO I, AF320026; Sb H 0 2 , AF320027), maize (Zm H O I , EST AWO I7939), tomato (Le
H O I , AF320028; Le H 0 2 , AF320029) and tobacco (Nt H O I , partial cDNA sequence AF473906). Bacterial HOs: Pseudomonos oeruginoso
(Po BphO, A83 I3 I), Deinococcus rodrodurons (Dr BphO, AF3967 lo), Corynebocterium diphtherioe (Cd HmuO, U73860) and Streptomyces
coelrcolor (Sc HO, AL 133220). Algal (plastid genome) and cyanobacterial HOs: Synechosystis PCC6803 (Syns HO I , D9090 I : Syns H 0 2 ,
D909 I2), Synechococcus PCC7942 (Sync H O I , AF048758), Guilordro theta (Gt PbsA, NC-000926), Porphyro purpureo (Pp PbsA,
NC-000926), Rhodello violoceo (Rv PbsA, AH005544) and Prochlorococcus morinus (Pm H O I , AY030299). Animal HOs: rat (Rn H O I ,
NM-0 12580; Rn H 0 2 , NM-024387), human (Hs H O I , NM-002 133; Hs H 0 2 , XM-036680) chicken (Gg HO I , P I479 I ) and Fugu (Jr
H O I , AF0228 14). Note that H O I and H 0 2 nomenclature for plant, animal and cyanobacterial HOs does not imply any functional or
greater sequence similarity between these proteins, and defines only separate groups within each phylum.
Plant
Bacte,rial
ibac'ter.id/
istid
01
Animal
0 2002 Biochemical Society
606
ec:nome)
Tetrapyrroles: Their Life, Birth and Death
preliminary biochemical evidence suggests that
plant H O l s are more similar to their cyanobacterial and algal counterparts than they are to
the mammalian enzymes. Arabidopsis H 0 1 is a
soluble protein that is able to utilize ferredoxin
and a ferredoxin-NADP+ oxidoreductase in vitro
[22]. These properties have also been seen for the
cyanobacterial [3,4] and algal [6,16,27] enzymes.
In contrast, mammalian H O uses NADPHcytochrome P450 reductase [2] and is membranebound through a hydrophobic C-terminal extension [28] that is absent from plant HOs [22,23,25].
Other mechanistic features of the algal and cyanobacterial enzymes, such as a dependence on a
second reductant such as ascorbate and a requirement for an iron chelator [3,29], are also shared by
Arabidopsis H 0 1 ( T . Muramoto, NI. J. Terry,
A. Yokota and T. Kohchi, unpublished work).
T h e cofactor requirements of the other HOs have
yet to be determined. Indeed, it has yet to be formally established whether they have H O activity
at all. This is particularly pertinent for H02, as it
lacks the conserved histidine that is thought to be
crucial for H O function (see above).
mutants with long hypocotyls in white light [35].
Further analysis of this mutant suggests that it is
almost completely ‘blind’ to red and far-red light
and contains no detectable holophytochrome
[25,35,36], indicating that at this developmental
stage H01 is the H O primarily responsible for
POB synthesis. However, despite this apparently
clear-cut observation, mutants deficient in H 0 2
do show a small decrease in holophytochrome and
a parallel (small) decrease in light responsivity,
suggesting that H 0 2 does have a role in POB
synthesis in seedlings [25]. This is supported by
the observation that HO2 is expressed throughout the seedling, as indeed is HOI [25]. So what
of H 0 3 and H 0 4 ? One of the characteristics of
all chromophore-deficient mutants is that they
‘recover’ as they mature [19]. One possibility is
that HOs other than H 0 1 play increasingly
important roles later in development. Indeed, the
ho2 mutant has an early-flowering phenotype that
appears to be somewhat stronger relative to hyl
than for comparative seedling responses [25].
It is possible that H 0 3 and H 0 4 also have a
more prominent role at other developmental
st ages.
An alternative hypothesis is that these additional HOs have different cellular locations and
that their primary function is not POB synthesis,
but haem degradation. T h e cytoplasm and mitochondria, for example, will have large haem pools,
and it might seem unlikely that haem would be
imported into plastids for degradation. In the absence of H 0 1 , BV IXa synthesized outside the
plastid may be utilized for POB synthesis, as has
been seen with the application of exogenous BV
IXa [36]. I n this context it is interesting that a
cytoplasmically localized rat H 0 1 could partially
rescue a HO-deficient mutant of the moss Ceratodon [37]. While this is an attractive explanation,
preliminary experiments with green fluorescent
protein constructs indicate that H 0 2 , H 0 3 and
H 0 4 proteins are all localized predominantly to
the plastid ( T . Kohchi, unpublished work). These
results would not preclude a dual localization
within the cell, but do suggest that another
explanation for the multiplicity of HO isoforms is
required. Another possibility, as suggested by
Davis et al. [25], is that H 0 3 and H 0 4 may have
tissue-specific roles. In this case it was noted that
H 0 l and H 0 2 were not strongly expressed in root
tips of etiolated seedlings, a region in which
phytochrome genes are highly expressed. Localization studies on H 0 3 and HOJ expression will
hopefully resolve this issue.
Role of HO in phytochrome synthesis
T h e phytochromes comprise a family of photoreceptors (there are five in Arabidopsis) that modulate growth and development in response to
changes in the surrounding light environment
[30]. They respond primarily to red and far-red
light and are photoreversibly regulated via the
covalently bound chromophore, POB. T h e pathway leading to phytochrome holoprotein synthesis
is depicted in Figure 2. Phytochrome genes are
nuclear-encoded, and the apoproteins are synthesized on cytoplasmic ribosomes. In contrast, both
H O [22] and the next enzyme in the pathway, POB
synthase, are plastid-localized [31,32], indicating
that the complete POB synthetic pathway is
present in plastids (see also [7]). T h e primary
product of POB synthase is actually the 32-isomer
of POB [32,33], while 3E-POB is believed to be the
immediate precursor of the bound chromophore
[7]. A POB isomerase to accomplish this reaction,
if present, has yet to be identified. Assembly of
apophytochrome to POB is autocatalytic [34] and
is currently thought to take place in the cytoplasm
[7]. Once assembled, phytochrome transduces its
signal through both cytoplasmically and nuclearlocalized signalling components, and is itself transported to the nucleus [30].
T h e H 0 1 -deficient hyl mutant of Arabidopsis
was originally identified from a genetic screen for
607
0 2002 Biochemical Society
Biochemical Society Transactions (2002) Volume 30, part 4
Figure 2
Role of HO in the synthesis ofthe phytochrome family of plant photoreceptors
The phytochromes are synthesized from two components. The apoproteins are encoded by
nuclear genes and the chromophore, PQB, is synthesized in the plastid from 5-aminolaevulinic
acid ( A M ) via the haem and BV branch of the tetrapyrrole pathway. Assembly of
holophytochrome is autocatalytic and is thought to occur in the cytoplasm.
3E-POB
r
red/far-red
light
RESPONSES
HOLOPHYTOCHROME
One example where the role of HOs in
phytochrome chromophore synthesis is well defined comes from the recent identification of HO
genes on the same operons as genes encoding
bacteriophytochromes [38]. This observation is
made even more intriguing by the finding that
these bacteriophytochromes, present in heterotrophic bacteria such as Deinococcus, appear to use
BV IXa itself as their chromophore [38].
can lead to an inhibition of chlorophyll synthesis
through a feedback effect on 5-aminolaevulinic
acid production, which is rate-limiting for tetrapyrrole synthesis [40]. This in turn can lead to
effects on plastid development [41] and has been
proposed as an explanation for the extremely pale
phenotype of h y l [19].
T h e h y l mutant has also been identified in a
screen for genomes uncoupled (gun) mutants, in
which the control of nuclear gene expression by
the developmental status of the plastid has been
disturbed [42,43]. Mutants deficient in H01 ( h y l )
and the next enzyme in the pathway, PQB synthase
(hyZ), turned out to be allelic to gun2 and gun3
respectively [43], and these mutations are likely to
affect plastid development, through disruption of
the normal regulatory mechanisms mediated by
plastid signalling pathways. There is increasing
evidence that tetrapyrroles play an important role
in plastid signalling, and it has been proposed that
the signal may represent a balance between two
tetrapyrrole species [43,44]. It is therefore probable that h y l affects plastid signalling through
perturbation of the tetrapyrrole pathway, although
a more direct involvement cannot be ruled out.
Do HOs have additional roles in
plastid development?
In addition to being elongated, the h y l mutant is
also pale in colour due to defects in normal
chloroplast development, consistent with the
known roles of phytochromes in this aspect of
development [39]. However, despite retaining
some phytochrome responses, h y l is paler than a
quadruple mutant lacking phytochromes A, B, D
and E (G. Whitelam, personal communication).
This suggests that H O deficiency affects chloroplast development in a manner independent of the
absence of phytochrome. Analysis of tetrapyrrole
synthesis in chromophore mutants of tomato has
shown that a block in plastidic haem degradation
0 2002 Biochemical Society
608
Tetrapyrroles: Their Life, Birth and Death
These phenotypic traits represent an extreme
situation, i.e. a complete absence of H01. Is it
possible that regulation of H 0 1 expression has a
role in modulating plastid development? Initial
studies on the regulation of H 0 1 mRNA levels
suggest that this is not a highly regulated gene,
with only a 2-fold increase in light-grown compared with dark-grown Arabidopsis seedlings
[23]. A small increase such as this would be in
keeping with an increased requirement for haem
turnover in the light. However, it is equally likely
that, if H01 were to play an important regulatory
role, it would be responsive to the flux through the
haem branch of the tetrapyrrole pathway, perhaps
through modulation of its activity or abundance.
These questions still remain to be addressed, but
we are now at least in a position to do so.
17
18
19
20
21
22
23
24
25
26
27
Work on HOs is supported by BBSRC grant 5 I /P I0948 to M.J.T.,
a grant from the Japanese Society for the Promotion of Science
'Research for the Future' program (JSPS-RFTFOOLO 1605) to
T.K.. and a BBSRC lSlS award (ISIS 982) to M.J.T.and T.K. M.J.T.
is a Royal Society University Research Fellow. Thanks also to
Professor Peter Shoolingin-Jordan (University of Southarnpton)
for reading the manuscript.
28
29
30
31
32
References
I
2
3
4
5
6
7
8
9
10
II
I2
13
14
I5
16
Tenhunen, R., Marver, H. S.and Schrnid, R. ( I 968)
Proc. Natl. Acad. Sci. U S A 6 I, 748-755
Ortiz de Montellano, P. R. and Wilks, A. (200 I ) Adv. Inorg
Chern. 5 I,359407
Comejo, J. and Beale, S.I. ( 1997) Photosynth. Res. 5 I ,
223-230
Comejo, J., Willows, R. D. and Beale, S.I. ( I 998) Plant J.
15, 99- I07
Wilks, A. and Schrnitt, M. P. ( I 998) J. Biol. Chern. 273,
837-84 I
Beale, S. I. ( 1993) Chern. Rev. 93,785-802
Terry, M. J., Wahleithner, J. A. and Lagarias,J, C. ( I 993)
Arch. Biochern. Biophys. 306, 1-15
Falchuk K. H., Contin, J, M., Dziedzic, T. S.,Feng. Z., French,
T. C., Heffron, G.J. and Montomi, M. (2002) Proc. Natl.
Acad. Sci. U.S.A.99, 25 1-256
Stocker, R., Yamarnoto, Y., McDonagh, A. F., Glazer, A. N.
and Arnes, B. N. ( 1987) Science 235, 1043- I046
Platt, J. L. and Nath. K. A. ( I 998) Nat. Med. (N.Y.) 4,
I 363- I 365
Vile, G. F., Basu-Modak. S., Waltner, C. and Tyrell, R. M.
( 1994) Proc. Natl. Acad. Sci. U.S.A. 9 I, 2607-26 I0
Richaud, C. and Zabulon, G. ( 1997) Proc. Natl. Acad.
Sci. U.S.A. 94, I 1736- I I 74 I
Maines, M. D. ( 1997) Annu. Rev. Phamacol. Toxicol. 37,
5 17-554
Troxler, R. F.. Brown, A. S. and Brown, S.B. ( I 979) J. Biol.
Chern. 254, 34 I 1-34 I 8
Beale, S. I. and Comejo, J. ( I 984) Arch. Biochern. Biophys.
235, 37 1-384
Comejo, J. and Beale, S.I. ( I 988) J. Biol. Chem. 263,
11915-1 1921
33
Elich, T. D., McDonagh, A. F., Palma, L. A. and Lagarias,J. C.
( I 989) J. Biol. Chern. 264, 183-1 89
Konomi, K., Li, H.-S., Kuno, N. and Furuya, M. ( I 993)
Photochem. Photobiol. 58, 852-857
Terry, M. J. ( 1997) Plant Cell Environ. 20, 740-745
Weller, J. L., Terry, M. J., Rameau, C., Reid, J, B. and
Kendrick, R. E. ( I 996) Plant Cell 8. 55-67
Terry, M. J. and Kendrick. R. E. ( I 996) J. Biol. Chem. 271.
21681-21686
Murarnoto, T., Kohchi, T., Yokota, A,, Hwang, I. and
Goodman, H. M. ( 1999) Plant Cell I I , 335-347
Davis, S.J., Kurepa, J. and Vientra, R. D. ( I 999) Proc. Natl.
Acad. Sci. U.S.A. 96, 654 1-6546
The Arabidopsis Genome Initiative (2000) Nature
(London) 408,796-8 I5
Davis, S.J., Bhoo, S.-H.,Dunki, A. M., Walker, J. M. and
Vierstra, R. D. (2001) Plant Physiol. 126, 656-669
Izawa. T., Oikawa, T.. Tokutomi. S..Okuno. K. and
Shimarnoto, K. (2000) Plant J. 22, 39 1-399
Rhie, G. and Beale. S.I. (1992) J. Biol. Chem. 267,
16088-16093
Schuller, D.J., Wilks, A., Ortiz de Montellano, P. R. and
Poulos, T. L. ( 1999) Nat. Struct. Biol. 6, 86G867
Rhie, G. and Beale, S.I. ( 1995) Arch. Biochern. Biophys.
320, 182-194
Smith, H. (2000) Nature (London) 407, 585-59 I
Terry, M. J. and Lagarias,J. C. ( I99 I ) J. Biol. Chem. 266,
222 I 5-22 22 I
Kohchi. T., Mukougawa. K., Frankenberg, N.. Masuda. M..
Yokota, A. and Lagarias.J. C. (200 I) Plant Cell 13,
425436
Terry, M. J., McDowell, M. D. and Lagarias,J. C. ( 1995)
J. Biol. Chem. 270, I I I I I I I I I9
Lagarias,J. C. and Lagarias, D. M. ( 1989) Proc. Natl. Acad.
Sci. U.S.A.86, 5778-5780
Koomneef, M., Rolff, E. and Spruit, C. J. P. ( 1980)
Z. Pflanzenphysiol. 100, 147- I 60
Parks, B. M. and Quail, P. H. ( I 99 I ) Plant Cell 3,
1177-1 186
Brijcker, G., Zeidler, M., Kohchi, T., Hartrnann. E. and
Larnparter, T. (2000) Planta 210, 529-535
Bhoo, S.-H., Davis, S.J., Walker, J., Kamiol, B. and Vientra,
R. D. (2001) Nature (London) 414, 776-779
Chory, J., Peto, C. A., Ashbaugh, M., Saganich, R., Pratt, L.
and Ausubel, F. ( 1989) Plant Cell I, 867-880
Terry, M. J. and Kendrick, R. E. ( I 999) Plant Physiol. I 19,
143- I52
Terry, M. J., Ryberg, M.. Raitt. C. E. and Page, A. (200 I )
Planta 214. 3 14-325
Susek R. E.. Ausubel. F. M. and Chory, J. ( I 993) Cell 74,
787-799
Mochizuki, N.,Brusslan, J. A., Larkin, R., Nagatani, A. and
Chory, J. (200 I ) Proc. Natl. Acad. Sci. U.S.A. 98,
2053-2058
Vinti, G., Hills, A., Campbell, S., Bowyer, J. R.. Mochizuki, N..
Chory, J. and Lopez-Juez.E. (200 I ) Plant J, 24, 883-894
Thompson, J. D., Higgins, D. G. and Gibson, T. J. ( I 994)
Nucleic Acids Res. 22. 4673-4680
-
34
35
36
37
38
39
40
41
42
43
44
45
Received I I March 2002
609
0 2002 Biochemical Society