240
Plant sulfur metabolism — the reduction of sulfate to sulfite
Julie Ann Bick and Thomas Leustek∗
Until recently the pathway by which plants reduce activated
sulfate to sulfite was unresolved. Recent findings on two
enzymes termed 5′-adenylylsulfate (APS) sulfotransferase and
APS reductase have provided new information on this topic.
On the basis of their similarities it is now proposed that these
proteins are the same enzyme. These discoveries confirm that
the sulfate assimilation pathway in plants differs from that in
other sulfate assimilating organisms.
Addresses
Biotechnology Center for Agriculture and the Environment, Rutgers
University, 59 Dudley Road, Foran Hall, New Brunswick, New Jersey
08901-8520 USA
∗e-mail: LEUSTEK@aesop.rutgers.edu
Current Opinion in Plant Biology 1998, 1:240–244
http://biomednet.com/elecref/1369526600100240
 Current Biology Ltd ISSN 1369-5266
Abbreviations
AK
5′-adenylyl sulfate kinase
APR
5′-adenylylsulfate reductase
APS
5′-adenylylsulfate
APSSTase 5′-adenylylsulfate sulfotransferase
PAPS
3′-phosphoadenosine-5′-phosphosulfate
matter of contention and is the focus of this review.
Skipping this step for the moment, the remaining reactions
in the pathway to cysteine include the reduction of
sulfite to sulfide catalyzed by ferredoxin-dependent sulfite
reductase [6], then assimilation of inorganic sulfide by
the sulfhydration of O-acetylserine [7••]. The second
assimilation pathway branching from APS is used for the
synthesis of a variety of sulfated compounds carried out by
specific sulfotransferases (ST’s). These enzymes use the
phosphorylated derivative of APS, 3-phosphoadenosine5′-phosphosulfate (PAPS)[4••,5••], formed by APS kinase
(AK).
This paper focuses on the latest information on the
enzyme catalyzing the reduction of APS. The recent
purification of APSSTase from a marine alga provides the
first evidence on the catalytic properties of this enzyme. At
the same time, cDNAs have been cloned from the higher
plant Arabidopsis thaliana that encode an enzyme termed
APS reductase that shows similarities to APSSTase. We
propose that they are the same enzyme and discuss their
properties in relationship to a role in sulfate assimilation.
Figure 1
Introduction
This article focuses on a single aspect of a broad subject
area. It is prompted by the recent findings concerning
one of the primary steps in the sulfate assimilation
pathway of higher plants. Readers who are interested in
a comprehensive treatment of plant sulfur metabolism are
referred to several recent reviews [1•–3•].
Higher plants are dependent on inorganic sulfate to fulfil
their nutritional sulfur requirement. Sulfate is a major
anionic solute and is required for the synthesis of many
different metabolites and coenzymes. After uptake from
the soil, sulfate is either accumulated and stored in the
vacuole or it is incorporated into organic compounds
in a process referred to as assimilation. Sulfur can be
assimilated in one of two ways — it is either incorporated
as sulfate in a reaction termed sulfation [4••,5••], or it is
first reduced to sulfide, the substrate for cysteine synthesis.
In plants the majority of sulfur is assimilated in the
reduced form.
The plant sulfate assimilation pathways are depicted
in Figure 1. The prerequisite step for all metabolic
fates of sulfate is its activation by linkage to 5′-AMP,
forming 5′-adenylylsulfate (APS), a reaction catalyzed
by ATP sulfurylase. APS lies at a metabolic branch
point — it is used either for sulfation or for the reduction
pathway. It is the first reduction step that has been a
NADP+
NADPH H+
O-acetylserine
GR
GSSG
2–
SO 4
APS
GSH
2–
SO 3
APR
(APSSTase)
2–
S
SiR
Cysteine
OASTL
AS
AK
Sulfated compounds
PAPS
ST's
Current Opinion in Plant Biology
The sulfate assimilation pathway in plants. Sulfate is activated by
ATP sulfurylase (AS) forming 5′-adenylylsulfate (APS). APS is a
branch point intermediate that is used for the reduction pathway
leading to formation of cysteine (upper pathway) or the sulfation
pathway leading to synthesis of various sulfated compounds (lower
pathway). The sulfation pathway begins by phosphorylation of APS
to 3′-phosphoadenosine-5′phosphosulfate (PAPS) by APS kinase
(AK). PAPS is the substrate for a variety of sulfotransferases (ST’s).
In the cysteine pathway, the first reduction step is the topic of this
article. We argue that the enzyme APS reductase (APR) is identical
to APS sulfotransferase (APSSTase), a glutathione-dependent
reductase. Its activity is driven by reduced glutathione (GSH) which
is recycled from oxidised glutathione (GSSG) through the action
of NADPH-dependent glutathione reductase (GR). The subsequent
reactions are the reduction of sulfite to sulfide by sulfite reductase
(SiR) and assimilation of sulfide into cysteine by O-acetylserine (thiol)
lyase (OASTL).
Plant sulfur metabolism — the reduction of sulfate to sulfite Bick and Leustek
Sulfate reduction in plants
The long-held dogma concerning sulfate reduction stated
that, in plants, APS is the substrate for this reaction
and the enzyme APS sulfotransferase (APSSTase) [8].
For a number of reasons, however, the APS pathway
in plants was not universally accepted [9] — despite
years of research, definitive evidence for the existence
of APSSTase was not forthcoming. One report of its
purification from Euglena gracilis was confounded by the
unreasonably low specific activity of the pure enzyme
[10]. Subsequently, another group reported that, in vitro,
plant APS kinase displays APS sulfotransferase activity
as a side reaction [11]. This result, combined with
several physical similarities, prompted the idea that
perhaps APS sulfotransferase is a kinetic artefact [11].
In contrast to this uncertainty, it is widely believed that
prokaryotes (including cyanobacteria) and fungi use PAPS
for sulfate reduction via the enzyme PAPS reductase
[12,13]. Recently, however, direct evidence was published
by three independent groups. confirming the existence of
an APS-dependent pathway in plants [14••–16••].
APS sulfotransferase
Although APSSTase has long been the focus of investigation [17] its purification to homogeneity proved to
be an elusive goal. Kanno et al. [14••] finally succeeded
in isolating the enzyme from a marine macroalga by
maintaining high levels of ammonium sulfate throughout
the purification procedure. Previous publications noted the
lability of APSSTase and that it could be stabilized with
high concentrations of sulfate salts [18], but this finding
was never incorporated into a purification scheme.
As reflected by its name, APSSTase was proposed to
catalyze the transfer of sulfate from APS to a thiol acceptor
molecule forming a thiosulfonate [8]. In vitro, many different reduced thiol compounds, including dithiothreitol,
can serve as the acceptor. The physiological acceptor was
envisaged to be glutathione, primarily because it is an
efficient substrate and is the most abundant thiol in the
chloroplast stroma in which APSSTase is localized [19].
The algal enzyme shows a kinetic constant for glutathione
of 0.6 mM, a value that is well below the physiological
concentration in the stroma, reported to be between 3 and
10 mM [20,21]. This is a key finding that supports much of
the earlier work on this enzyme. For example, it has long
been known that sulfite production from sulfate is not light
dependent, requiring only ATP and glutathione (it is not
influenced by NADPH or ferredoxin) [22].
Purified algal APSSTase has a high catalytic activity
(10.3 µmol min–1 mg–1 protein) and a high affinity for APS
(Km = 2.1 µM). It shows a high pH optimum (9.0–9.5)
and it is maximally active in the presence of high
sulfate concentrations (0.5 M). The enzyme is sensitive to
inactivation by heat and dithiothreitol, but not glutathione.
High concentrations of sulfate salts and APS (or APS
241
analogs) stabilize the enzyme against these inactivating
treatments.
Although the reaction mechanism of APSSTase has not
been addressed, this is a central issue that should be
explored with the purified enzyme. As already mentioned
APSSTase was proposed to be a sulfotransferase. For
example, with glutathione as the acceptor S-sulfoglutathione is formed. It was acknowledged, however, that
this hypothesis is formed on the basis of inconclusive
data and that it is equally plausible that the enzyme is
a reductase that forms free sulfite [8]. Due to its high
reactivity sulfite can readily form a thiosulfonate with
reduced thiols [23], possibly explaining the presence of
thiosulfonate in APSSTase reactions.
The purification of APSSTase from a macroalga is the
first direct and convincing evidence for an APS-dependent
reduction pathway in plants. Additional and complementary evidence for such a pathway was contemporaneously
obtained using molecular genetic methods as described in
the next section.
The APR cDNAs encoding APS reductase
APS reductase (APR) was identified serendipitously by
searching for cDNAs encoding plant PAPS reductase
[15••,16••]. Three different cDNAs were cloned from
Arabidopsis thaliana (APR1, 2 and 3) that are able to
complement the cysteine auxotrophy of an Escherichia coli
cysH (PAPS reductase) mutant strain. The APR cDNAs
encode individual members of a small, highly conserved
gene family.
Analysis of the APR isoenzymes revealed that, unlike
PAPS reductase, they use APS preferentially over PAPS.
This property was demonstrated both by in vitro assays
and by in vivo functional complementation of a cysC
(APS kinase) mutant of E. coli with the APR cDNAS.
An APS reductase would be expected to bypass both
the cysC and cysH mutations in E. coli. Another key
difference is that, while PAPS reductase is dependent
on the redox proteins thioredoxin or glutaredoxin as a
source of electrons, the activity of the APR enzymes is not
stimulated in vitro by the addition of E. coli or Spirullina sp.
thioredoxin. Also, the APR cDNAs are able to complement
a thioredoxin/glutaredoxin double mutant of E. coli.
The properties of the APR enzymes are remarkably similar
to those of APSSTase with respect to optimum assay
conditions, kinetic constants and inhibitors. A key feature
is that like APSSTase the APR enzymes are able to use a
variety of reduced thiol compounds such as dithiothreitol
and glutathione as a sole source of electrons. The enzyme
shows a Km for glutathione of 0.6 mM and can only
complement the cysH mutation of E. coli if the strain
produces glutathione (Bick and Leustek, unpublished
data). Although the APR enzymes have not yet been
directly linked with APSSTase the similarity of their
catalytic features is a strong argument that they are the
242
Physiology and metabolism
same enzyme. The derived amino acid sequences of the
APR cDNAs may reveal how APS reductase (and probably
APSSTase) might function with only glutathione as a
reductant. The APR enzymes are composed of three
domains (Figure 2). At the amino terminus is a region that
resembles a transit peptide; this would be cleaved from the
protein following import into the chloroplast. Adjacent to
this is the amino-terminal domain of the mature protein
that is homologous with PAPS reductases from a variety
of sulfate assimilating organisms. At the carboxyl end
is a domain that resembles thioredoxin. The fusion of
reductase and cofactor into a single protein suggests that
the thioredoxin-like domain may serve as an exclusive
electron donor for the reductase domain. But what about
the interaction with glutathione?
Evolution of APR enzymes
Figure 2
The evolution of the APR gene family in A. thaliana was
explored by analysis of its genomic sequences [28•]. Figure
3 shows that APR1 is encoded by three exons that are
separated by two introns. In contrast, APR2 and APR3
are composed of four exons that are separated by three
introns: the salient difference being that APR1 lacks the
intron that separates exons 2 and 3 in APR2 and APR3. At
the level of nucleotide homology, the coding sequence of
APR1 is more closely related to APR3 (78% identity) than
to APR2 (68% identity). This suggests that APR1 emerged
by duplication of an ancestral APR1/APR3 gene followed
by the loss of exon 2. The only PAPS reductase gene that
is known to be interrupted is from the filamentous fungus
Emericella nidulans (GenBank accession #X82555). This
gene contains a single intron that interrupts the coding
sequence at a position very close to the position of intron
2 in APR1 and intron 3 in APR2 and APR3.
TP
Reductase domain
trx/grx domain
Current Opinion in Plant Biology
Domain structure of the APR enzymes. The APR enzymes are
composed of three domains. The amino terminal domain is a
transit peptide (TP) required for localization to chloroplasts and
is proteolytically removed to form the mature enzyme. The mature
enzyme is composed of a reductase domain (shaded dark)
homologous with bacterial PAPS reductase, and a carboxyl terminal
domain (shaded light) homologous with thioredoxin (trx).
Thioredoxin and related proteins are redox-active proteins
that function with a number of different reductases
[24,25]. A hallmark of these proteins is an active site
consisting of two cysteine residues, spaced two amino
acids apart, that switch between the dithiol and disulfide.
Plants contain thioredoxin proteins in both the cytoplasm
and chloroplast and these are reduced by NADPH-dependent or ferredoxin-dependent thioredoxin reductases. The
closely related protein, glutaredoxin, is reduced by glutathione, which is itself reduced by NADPH-dependent
glutathione reductase. The natural redox systems used
by these proteins are highly specific. For example, in
contrast to glutaredoxin, thioredoxin is unable to accept
electrons from glutathione. Interestingly, although the
carboxyl domain sequence shows greater overall homology
with thioredoxin, the amino acid residues between the
active site cysteines are more typical of glutaredoxin. It
has been demonstrated that these amino acids influence
the redox potential of the cysteines and, therefore, the
redox system that the proteins can react with [26]. Indeed,
we have shown that the carboxyl domain functions more
like glutaredoxin than thioredoxin. The implication of this
result is that sulfate reduction in plants may ultimately be
driven by NADPH through the activity of a glutathione
reductase as depicted in Figure 1.
Despite the difference in substrate specificity the APR
enzymes are clearly related in sequence to PAPS reductase
from prokaryotes and fungi, but they are not related to
APS reductase from dissimilatory sulfate reducing bacteria;
this enzyme is a heteromeric iron-sulfur flavoprotein [27].
To avoid confusion, we propose that the plant enzyme
be referred to as the plant-type APS reductase. A recent
search of the DNA sequence databases revealed that
PAPS reductase from Pseudomonas aeruginosa (GenBank
accession #U95379) and Rhizobium tropici (AJ001223) has
the highest homology with the reductase domain of the
APR enzymes (∼70% amino acid similarity), far greater
than that with PAPS reductase from enteric bacteria,
cyanobacteria or fungi (with which the reductase domain
shares only ∼50% amino acid similarity).
A question of key significance in the evolution of
the APR enzymes is how three distinct domains — the
plastid transit peptide, the reductase domain, and the
thioredoxin/glutaredoxin domain — became fused into a
single polypeptide. One popular idea on gene evolution
is that the domains of proteins arise through the accretion
of separate exons [29]. If this were the case in APR
evolution one might expect to find an intron separating
the exons encoding individual domains. This appears
to be the case with respect to the transit peptide. In
all three genes it is encoded by exon 1 although the
protease cleavage site is encoded at the beginning of exon
2. In contrast, the other domains are not separated by
an intron, rather a portion of the reductase domain and
the thioredoxin/glutaredoxin domain are fused into single
exon. This finding does not help to clarify the evolutionary
origin of the gene. An interesting point, however, is that
in APR1 there is a nearly perfect 12 basepair sequence
duplication located at the 5′ end of intron 2 (gtatatttcgat)
and another within exon 3 (gtatatatcgat) at a position
that is approximately at the border between the reductase
and thioredoxin/glutaredoxin-encoding sequences. Similar,
but less well conserved sequences, are present at the
Plant sulfur metabolism — the reduction of sulfate to sulfite Bick and Leustek
243
may provide a basis for regulation of enzyme level, namely
that changes in gene expression alter the protein level.
Figure 3
Conclusions
APR1
APR3
APR2
Current Opinion in Plant Biology
Exon Intron structure of the APR genes. The genomic sequence
from the translational initiation codon to the termination codon is
shown as a diagram. The solid bars reflect the coding exons and the
open bars reflect introns. The dotted lines indicate the analogous
introns in each gene. The solid circle shows the position of the
12 basepair direct repeat sequences. The open circle shows the
approximate border between the sequence encoding the reductase
and thioredoxin/glutaredoxin domains.
The next few years promise to be an exciting time
for the community of plant sulfur researchers. Gene
cloning has emerged as a powerful tool to define the
genes and enzymes of the sulfur assimilation pathways
in plants and the effort of cataloging genes continued in
1997 [33•,34••,35••36•]. The full potential of molecular
methods, however, has yet to be realized. We expect that
soon the biochemistry, enzymology and cell biology of
the sulfur assimilation pathway will be known at a level
of detail that will rival what is known about nitrogen
assimilation.
Note added in proof
The paper referred to in the text as (Bick and Leustek,
unpublished data) has now been accepted for publication
[37••].
References and recommended reading
same position in APR2 and APR3. It is an intriguing
possibility that this repeat is the remnant of an intron
that divided the last exon of these genes, isolating the
thioredoxin/glutaredoxin-like domain in a separate exon.
Contribution of APR gene expression to the
regulation of sulfate assimilation
Although the sulfate assimilation pathway in plants
appears to be regulated, the published evidence is not very
clear on which steps are most important. This is in contrast
to the situation in E. coli where the genes encoding all
the pathway enzymes are co-ordinately regulated by gene
expression in response to the availability of cysteine [13].
Thus, we must consider whether regulation in plants
might be highly co-ordinated at the level of tissues and
cells rather than at the whole plant level. Recently, the first
attempt at defining the specific cells and tissues in which
sulfate assimilation genes are expressed was published
[30••]. In the future, more work of this kind is needed.
APSSTase has long been the focus of investigation
because it is believed to be a key point for regulation
of the sulfate assimilation pathway. Most importantly, the
published literature indicates that changes in enzyme
level are the primary means by which APSSTase activity
is regulated [31]. Recent investigations on the affect of
sulfate starvation on A. thaliana showed that the steady
state level of mRNA for sulfate permease and the APR
enzymes increases markedly in roots and to a lesser extent
in leaves [32•]. The mRNA level for all of the other sulfate
assimilation genes was not increased or, in some cases,
it decreased. This result is consistent with the idea that
APSSTase and APS reductase are the same enzyme and
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
Brunold C, Rennenberg H: Regulation of sulfur metabolism in
•
plants: first molecular approaches. Prog Bot 1997, 58:164-186.
An excellent recent review on the topic of plant sulfur metabolism.
2.
Hell R: Molecular physiology of plant sulfur metabolism. Planta
•
1997, 202:138-148.
See annotation [1•].
3.
•
Schwenn J: Assimilatory reduction of inorganic sulphate. In
Sulphur Metabolism in Higher Plants: Molecular, Ecophysiological
and Nutritional Aspects. Edited by Cram WJ, De Kok LJ, Stulen I,
Brunold C, Rennenberg H. Leiden: Backhuys Publishers; 1997:3958.
See annotation [1•].
4.
••
Varin L, Chamberland H, Lafontaine JG, Richard M: The enzyme
involved in sulfation of the turgorin, gallic acid 4-O-(β-Dglucopyranosyl-6′-sulfate) is pulvini-localized in Mimosa
pudica. Plant J 1997, 12:831-838.
An important contribution to the field of sulfation in plants, a poorly understood area of sulfate metabolism.
5.
••
Varin L, Marsolais F, Richard M, Rouleau M: Biochemistry and
molecular biology of plant sulfotransferases. FASEB J 1997,
11:517-525.
See annotation [4••].
6.
Brühl A, Haverkamp T, Gisselmann G, Schwenn JD: A cDNA clone
from Arabidopsis thaliana encoding plastidic ferredoxin:sulfite
reductase. Biochem Biophys Acta 1996, 1295:119-124.
7.
••
Bogdanova N, Hell R: Cysteine synthesis in plants: proteinprotein interactions of serine acetyltransferase from
Arabidopsis thaliana. Plant J 1997, 11:251-262.
Not discussed in our review is the topic of complex formation between Oacetylserine (thiol) lyase and serine acetyltransferase, which together form
cysteine synthetase. The function of this complex is not understood. This is
the first attempt to define the complex at the molecular level.
8.
Schmidt A, Jäger K: Open questions about sulfur metabolism
in plants. Annu Rev Plant Physiol and Plant Molec Biol 1992,
43:325-349.
9.
Schwenn JD: Photosynthetic sulfate reduction. Z Naturforsch
1994, 49:531-539.
244
Physiology and metabolism
10.
Li J, Schiff JA: Purification and properties of adenosine 5′phosphosulfate sulfotransferase from Euglena. Biochem J
1991, 274:355-360.
11.
Schiffmann S, Schwenn JD: APS-sulfotransferase activity is
identical to higher plant APS-kinase (EC 2.7.25). FEBS Lett
1994, 355:229-232.
12.
Berendt U, Haverkamp T, Prior A, Schwenn JD: Reaction
mechanism of thioredoxin: 3′-phospho-adenylylsulfate
reductase investigated by site-directed mutagenesis. Eur J
Biochem 1995, 233:347-356.
13.
Kredich NM: Biosynthesis of cysteine. In Escherichia coli and
Salmonella. Cellular and Molecular Biology. Edited by Neidhardt
FC. Washington DC: ASM Press; 1996:514-527
14.
••
Kanno N, Nagahisa E, Sato M, Sato Y: Adenosine 5′phosphosulfate sulfotransferase from the marine macroalga
Porphyra yezoensis Ueda (Rhodophyta): stabilization,
purification, and properties. Planta 1996, 198:440-446.
This reference is the first convincing purification of APS sulfotransferase.
15.
••
Setya A, Murillo M, Leustek T: Sulfate reduction in higher plants:
molecular evidence for a novel 5-adenylylphosphosulfate
(APS) reductase. Proc Natl Acad Sci USA 1996, 93:1338313388.
References [15••] and [16••] simultaneously described the isolation of APS
reductase cDNAs. The information in these papers confirms the existence
of an APS dependent sulfate reduction pathway in plants.
16.
••
Gutierrez-Marcos JF, Roberts MA, Campbell EI, Wray JL: Three
members of a novel small gene-family from Arabidopsis
thaliana able to complement functionally an Escherichia coli
mutant defective in PAPS reductase activity encode proteins
with a thioredoxin-like domain and ‘APS reductase’ activity.
Proc Natl Acad Sci USA 1996 93:13377-13382.
See annotation for [15••].
17.
Schmidt A: On the mechanism of photosynthetic sulfate
reduction. An APS sulfotransferase from Chlorella. Arch
Microbiol 1972, 84:77-86.
18.
Schmidt A: A sulfotransferase from spinach leaves using
adenosine-5′-phosphosulfate. Planta 1975, 124:267-275.
19.
Tsang ML-S, Schiff JA: Studies of sulfate utilization by algae.
18. Identification of glutathione as a physiological carrier in
assimilatory sulfate reduction by Chlorella. Plant Sci Lett 1978,
11:177-183.
20.
Anderson JW, Foyer CH, Walker DA: Light-dependent reduction
of hydrogen peroxide by intact spinach chloroplasts. Biochem
Biophys Acta 1983, 724:69-74.
21.
Foyer CH, Halliwell B: The presence of glutathione and
glutathione reductase in chloroplasts: a proposed role in
ascorbic acid metabolism. Planta 1976, 133:21-25.
22.
Schmidt A, Trebst A: The mechanism of photosynthetic sulfate
reduction by isolated chloroplasts. Biochim Biophys Acta 1969,
180:529-535.
23.
Würfel M, Häberlein I, Follman H: Inactivation of thioredoxin by
sulfite ions. FEBS 1990, 268:146-148.
24.
Buchanan BB, Schürmann P, Decottignies P, Lozano RM:
Thioredoxin: A multifunctional regulatory protein with a bright
future in technology and medicine. Arch Biochem Biophys
1994, 314:257-260.
25.
Holmgren A: Thioredoxin and glutaredoxin systems. J Biol
Chem 1989, 264:13963-13966.
26.
Krause G, Lundström J, Lopez Barea J, Pueyo de la Cuesta C,
Holmgren A: Mimicking the active site of protein disulfideisomerase by substituting proline 34 in Escherichia coli
thioredoxin. J Biol Chem 1991, 266:9494-9500.
27.
Speich N, Dahl C, Heisig P, Klein A, Lottspeich F, Stetter KO,
Trüpper HG: Adenylylsulphate reductase from the sulphatereducing archaeon Archaeoglobus fulgidus: cloning and
characterization of the genes and comparison of the enzyme
with other iron-sulphur flavoproteins. Microbiol 1994,
140:1273-1284.
28.
•
Chen Y, Leustek T: Three genomic clones from Arabidopsis
thaliana encoding 5′-adenylylsulfate reductase (Accession
Nos. AF016282, AF016283 and AF016284). Plant Physiol 1997,
116:447.
This paper provides information on the interesting topic of evolution of the
APR enzymes.
29.
Traut TW: Do exons code for structural or functional units in
proteins? Proc Natl Acad Sci USA 1988, 85:2944-2948.
30.
••
Gotor C, Cejudo FJ, Barroso C, Vega JM: Tissue-specific
expression of ATCYS-3A, a gene encoding the cytosolic
isoform of O-acetylserine(thiol)lyase in Arabidopsis. Plant J
1997, 11:347-352.
This is the first attempt to define the tissue and cellular specificity of the
expression of a sulfate assimilation gene.
31.
Brunold C, Suter M: Sulphur metabolism. B. Adenosine 5′phosphosulfate sulphotransferase. Methods Plant Biochem
1990, 3:339-342.
32.
•
Takahashi H, Yamazaki M, Sasakura N, Watanabe A, Leustek
T, de Almeida-Engler J, Engler G, Van Montagu M, Saito K:
Regulation of cysteine biosynthesis in higher plants: A sulfate
transporter induced in sulfate-starved roots plays a central
role in Arabidopsis thaliana. Proc Natl Acad Sci USA 1997,
94:11102-11107.
This is the first to attempt a systematic analysis of how the expression of
sulfate assimilation genes in plants is co-ordinated. In addition, information
on the sulfate permease gene family in Arabidopsis thaliana is presented.
33.
•
Howarth JR, Roberts MA, Wray JL: Cysteine biosynthesis in
higher plants: a new member of the Arabidopsis thaliana
serine acetyltransferase small gene-family obtained by
functional complementation of an Escherichia coli cysteine
auxotroph. Biochimica Biophysica Acta 1997, 1350:123-127.
These four papers [33•,34••,35••,36•] report the most recent sulfate assimilation genes to be cloned.
34.
••
Sakakibara H, Takei K, Sugiyama T: Isolation and
characterization of a cDNA that encodes maize
uroporphyrinogen III methyltransferase, an enzyme involved
in the synthesis of siroheme, which is a prosthetic group of
nitrite reductase. Plant J 1996, 10:883-892.
Papers [34••] and [35••] report for the first time on the gene and enzyme
involved in the synthesis of siroheme, the prosthetic group of sulfite and
nitrite reductase.
35.
••
Leustek T, Smith M, Murillo M, Singh DP, Smith AG, Woodcock
SC, Awan SJ, Warren MJ: Siroheme biosynthesis in higher
plants: analysis of an S-adenosyl-L-methionine-dependent
uroporphyrinogen III methyltransferase from Arabidopsis
thaliana. J Biol Chem 1997, 272:2744-2752.
See annotation [34••].
36.
•
Smith FW, Hawkesford MJ, Ealing PM, Clarkson DT, Vanden
Berg PJ, Belcher AR, Warrilow AGS: Regulation of expression
of a cDNA from barley roots encoding a high affinity sulfate
transporter. Plant J 1997, 12:875-884.
See annotation [33••]
37.
••
Bick JA, Aslund F, Chen Y, Leustek T: Glutaredoxin function
for the carboxyl terminal domain of the plant type 5′adenylylsulfate (APS) reductase. Proc Natl Acad Sci 1998, in
press.
This report shows that the two domains of APS reductase play interactive
but independent roles in catalysis and confirms that the carboxy-domain
functions as a glutaredoxin.