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(Last modification:
25. August 2010)
'Orphan' PKS in
Arabidopsis thaliana:
Alkylpyrone
Organization
of this page
1.
Orphan PKS from Arabidopsis thaliana: pyrone synthases
(Mizuuchi
et al., 2008)
With all the
work done on A. thaliana, you would think that understanding type III PKS
in this plant should present no problems. It does contain the gene for a
‘standard’ CHS (Feinbaum and
Ausubel, 1988), and it has long been thought that this was the only one (Burbulis
et al., 1996). The genome sequence showed that it is located on chromosome
five (DNA accession: At5g13930), but the comprehensive genome analysis also
revealed that A. thaliana contains three more genes for type III PKS: one
on chromosome one (At1g02050), and two on chromosome four (At4g00040 and
At4g34850). The functions of two of these (At1g02050 and At4g34850) were
recently analyzed with recombinant proteins (Mizuuchi et
al., 2008) (it was not quite clear to me why the third gene was not
investigated).
The in
vitro assays showed that both were not CHS or STS; there was no detectable
activity with the phenylpropanoid substrate investigated. However, there was
considerable activity with aliphatic fatty acid CoA-esters, with chain lengths
from four up to 20 carbon atoms. The majority of the products were the pyrones
from the triketides obtained after two condensations, but there was also a bit
of tetraketide pyrones, and a trace of phloroglucinols, i.e. the products from
three condensation reactions followed by a CHS-type ring-folding (see scheme
below). The proteins were then named PKS-A (At1g02050) and
PKS-B (At4g34850).
The properties, at least in vitro, were pretty similar, with one notable
exception: only PKS-A had some activity with acetyl-CoA (decarboxylation product
of the malonyl-CoA present as chain extender), with triacetic lactone (TAL) as
product, i.e. after two condensations.
These are
quite interesting findings, but unfortunately such pyrones have not yet been
described from A. thaliana, and thus both proteins should be defined as ‘orphan
PKS’ at this point. There is one set of data that might help in finding the
physiological function: apparently both genes are expressed in early developing
flower buds.
Nevertheless, it is noteworthy that these proteins are an interesting addition
to the growing number of type III PKS which substrate preferences for fatty acid
CoA-esters; these appear to be more wide-spread than thought before:
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Alkylresorcinole
und
langkettige Pyrone in dem Bakterium Azotobacter vinelandii:
Mehr...
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Alkylresorcinol-
Streptomyces
griseus: Mehr...
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Pyronsynthasen in dem Bakterium
Mycobacterium tuberculosis und
Bacillus subtilis: Mehr...
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CsyA: Pyronsynthasen
in dem Pilz Aspergillus oryzae:
Mehr...
Reactions of PKS-A and PKS-B from A. thaliana.
Boxed: the main products from fatty acid CoA-esters
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Update August 2010:
New
publication on these two proteins:
Dobritsa et al. (July 2010)
Just about two years later, another
group published another biochemical characterization of these two proteins,
called in this work LAP5 (=
PKS-B =
At4g34850) and LAP6 (=
PKS-B =
At1g02050) (LAP =
LESS ADHESIVE
POLLEN, the description of the
phenotype). The results were essentially the same: pyrones after two
condensations, also with 4-coumaroyl-CoA, the standard substrate for typical CHS
in naringenin chalcone biosynthesis.
However, this later work contains a lot more
experiments on the possible physiological roles of these proteins. Just a brief
summary, there are lots of interesting more results in the publication.
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The
expression is anther-specific, and inactivation of the genes leads to pollen
which is fertile, but has an aberrant exine patterning,
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Double mutants
abolish exine formation altogether, followed by pollen collapse and male
sterility,
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Analysis of the pollen shows that both mutants contained reduced amounts of
compounds requiring typical CHS activities, but they are not abolished
totally, and the LAP6 mutant even contained almost 6-fold higher amounts of
a quercetin derivative. There is really no evidence that these proteins have
CHS-functions in vivo,
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The
functions of LAP5 and LAP6 cannot be replaced by the typical CHS (TT4)
or by another CHS-like protein, encoded in At4g00040 (this has no
anther-specific expression),
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Like
shown already before, the
two proteins do have considerable activity in vitro with linear
CoA-esters (fatty acid CoA-esters), and thus it was possible that their
function is related to the formation of pyrones from fatty acids. However,
no candidate product for these activities could be identified in fairly
detailed metabolic profiling,
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Interestingly, the authors also tested whether combining LAP5 and LAP6 in
vitro would lead to new products, but that failed: there were
none. The approach was based on our previous finding that complementation of
two differently inactivated monomers can lead to active dimers (Tropf
et al., 1995), but the authors apparently overlooked an important point:
in those experiments the two proteins were co-expressed in vivo, and not
simply mixed in vitro after purification.
As stated above, this is only a brief summary.
What do we know now on the physiological / biochemical functions of these
genes/proteins in vivo? Unfortunately not much more than before, except
that we are sure now that these genes have functions in anther development.
2. A
subfamily of plant type III PKS with similar
functions?
The authors
of the A. thaliana publication (Mizuuchi et al., 2008) did some
relationship analysis, but the focus seemed to be a bit narrow. Most
interestingly, but not discussed: if you make a simple BLAST analysis with the
two A. thaliana proteins, you’ll find a fairly high number of pretty
closely related proteins in the data bases, with identities of 60% up to almost
80% with PKS-A and PKS-B from A. thaliana. Several of those sequences
resulted from genome sequencing, e.g. Populus trichocarpa (Tuskan
et al., 2006), grapevine (Vitis vinifera) (Jaillon
et al., 2007), corn (Zea mays) (Alexandrov
et al., 2009), and black cottonwood (Pinus radiata) (Walden
et al., 1999), and usually these studies did not worry about a functional
identification. But others were detected by analysis of tissue-specific
expression, e.g. in wheat (Triticum aestivum) and Triticale (Wu
et al., 2008), in rice (Oryza sativa) (Hihara
et al., 1996), Nicotiana sylvestris (Atanassov
et al., 1998), and Silene latifolia (Ageez
et al., 2005). Interestingly, many of these seem to be expressed in flower
formation/development, more specifically they appear to be anther-specific,
but the biochemical functions are not yet known, to the best of my knowledge.
Have a look at the relationship tree: all
these proteins form a distinct separate branch pretty remote from most other
plant type III PKS. In this context the functional identification of the A.
thaliana proteins may be quite relevant: Maybe the functions in this
subgroup are similar, for example with long-chain acyl-CoAs as starter
molecules? Maybe the products contribute to the lipids in these tissues.
It is also noteworthy that the ARS (alkylresorcinol synthase) from
Physcomitrella patens clusters fairly close as well, and that protein
clearly prefers long-chain acyl-CoAs as starter molecules (more…).
The most
interesting case, however, may be the PKS1 from Hypericum perforatum: it
is a likely candidate for the key reaction in Hyperforin biosynthesis: three
condensations with a short-chain aliphatic CoA-ester as starter, followed by a
CHS-type ring-folding (more…).
The figure below is a detail
from the general relationship tree: click here for a
full view.
Part of the general relationship tree of
type III PKS in plants.
Links
to other orphan type III PKS
Return to top
Citations
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Mizuuchi,
Y., Shimokawa, Y., Wanibuchi, K., Noguchi, H., Abe, I., 2008. Structure
function analysis of novel type III polyketide synthases from Arabidopsis
thaliana. Chemical & Pharmaceutical Bulletin (Tokyo) 31, 2205-2210.
The genome sequencing project revealed presence of two active
chalcone synthase (CHS) homologues (At1g02050 and At4g34850) in the model
plant Arabidopsis thaliana. We report herein the two genes encode
closely related novel plant-specific type III polyketide synthases (PKSs)
that produces long-chain alkyl alpha-pyrones. PKS-A (At1g02050) and PKS-B
(At4g34850) share significantly low amino acid sequence identity (20-40%)
with other type III PKSs, and the phylogenetic tree analysis revealed that
they form a separate cluster located closely to those of bacterial type III
PKSs. When expressed in Escherichia coli, both PKS-A and PKS-B
accepted unusually long (up to the C(20) chain-length) fatty acyl CoAs as a
starter substrate, and carried out sequential condensations with malonyl-CoA
to produce triketide and tetraketide alpha-pyrones. Interestingly, despite
the low sequence identity, homology modeling revealed that the active-site
architecture of PKS-A and PKS-B showed similarity to that of a bacterial
type III PKS from Mycobacterium tuberculosis.
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Dobritsa,
A. A., Lei, Z., Nishikawa, S., Urbanczyk-Wochniak, E., Huhman, D. V.,
Preuss, D., Sumner, L. W., 2010. LAP5 and LAP6 encode anther-specific
proteins with similarity to chalcone synthase essential for pollen exine
development in Arabidopsis. Plant Physiology 153, 937-955.
Pollen grains of land plants have evolved remarkably strong outer walls
referred to as exine that protect pollen and interact with female stigma
cells. Exine is composed of sporopollenin, and while the composition and
synthesis of this biopolymer are not well understood, both fatty acids and
phenolics are likely components. Here, we describe mutations in the
Arabidopsis (Arabidopsis thaliana) LESS ADHESIVE POLLEN (LAP5) and
LAP6 that affect exine development. Mutation of either gene results in
abnormal exine patterning, whereas pollen of double mutants lacked exine
deposition and subsequently collapsed, causing male sterility. LAP5 and LAP6
encode anther-specific proteins with homology to chalcone synthase, a key
flavonoid biosynthesis enzyme. Lap5 and lap6 mutations reduced the
accumulation of flavonoid precursors and flavonoids in developing anthers,
suggesting a role in the synthesis of phenolic constituents of
sporopollenin. Our in vitro functional analysis of LAP5 and LAP6
using 4-coumaroyl-coenzyme A yielded bis-noryangonin (a commonly reported
derailment product of chalcone synthase), while similar in vitro analyses
using fatty acyl-coenzyme A as the substrate yielded medium-chain alkyl
pyrones. Thus, in vitro assays indicate that LAP5 and LAP6 are
multifunctional enzymes and may play a role in both the synthesis of pollen
fatty acids and phenolics found in exine. Finally, the genetic interaction
between LAP5 and an anther gene involved in fatty acid hydroxylation
(CYP703A2) demonstrated that they act synergistically in exine production.
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Ageez, A., Kazama, Y., Sugiyama, R., Kawano, S.,
2005. Male-fertility genes expressed in male flower buds of Silene
latifolia include homologs of anther-specific genes. Genes & Genetic
Systems 80, 403-413.
When the female plant of Silene latifolia is infected with
the smut fungus Microbotryum violaceum, its rudimentary stamens
develop into anthers which contain fungus teliospores instead of pollen. To
identify genes required for maturation of anthers in S. latifolia, we
performed a cDNA subtraction approach with healthy male buds and female buds
infected with M. violaceum. We isolated five cDNA clones, which were
preferentially expressed in healthy male buds during stages associated with
a burst in tapetal activity. These five cDNAs are predicted to encode a
mandelonitrile lyase protein (SlMDL1), a strictosidine synthase protein (SlSs),
a glycosyl hydrolase 17 protein (SlGh17), a proline-rich protein APG
precursor (SlAPG), and a chalcone-synthase-like protein (SlChs). All
five genes showed expression in both healthy and fungus-infected male buds,
but not expressed in either healthy or infected female buds. The first three
genes were highly expressed in both tapetum and pollen grains while the last
two genes were expressed only inside the tapetum of male flower buds.
Phylogenetic analysis results showed that SlChs and SlGh17 belong to
anther-specific subgroups of chalcone-synthase-like genes and glycosyl
hydrolase 17 family genes, respectively. Our results suggest that the
isolated five genes are related to the fertility of the anther leading to
the development of fertile pollen. The smut fungus was not able to induce
the expression of the five genes in the infected female buds. This raises
the possibility that these genes are under the control of master gene(s) on
the Y chromosome.
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Alexandrov, N. N., Brover, V. V., Freidin, S.,
Troukhan, M. E., Tatarinova, T. V., Zhang, H., Swaller, T. J., Lu, Y. P.,
Bouck, J., Flavell, R. B., Feldmann, K. A., 2009. Insights into corn genes
derived from large-scale cDNA sequencing. Plant Molecular Biology 69,
179-194.
We present a large portion of the transcriptome of Zea mays,
including ESTs representing 484,032 cDNA clones from 53 libraries and 36,565
fully sequenced cDNA clones, out of which 31,552 clones are non-redundant.
These and other previously sequenced transcripts have been aligned with
available genome sequences and have provided new insights into the
characteristics of gene structures and promoters within this major crop
species. We found that although the average number of introns per gene is
about the same in corn and Arabidopsis, corn genes have more alternatively
spliced isoforms. Examination of the nucleotide composition of coding
regions reveals that corn genes, as well as genes of other Poaceae (Grass
family), can be divided into two classes according to the GC content at the
third position in the amino acid encoding codons. Many of the transcripts
that have lower GC content at the third position have dicot homologs but the
high GC content transcripts tend to be more specific to the grasses. The
high GC content class is also enriched with intronless genes. Together this
suggests that an identifiable class of genes in plants is associated with
the Poaceae divergence. Furthermore, because many of these genes appear to
be derived from ancestral genes that do not contain introns, this
evolutionary divergence may be the result of horizontal gene transfer from
species not only with different codon usage but possibly that did not have
introns, perhaps outside of the plant kingdom. By comparing the cDNAs
described herein with the non-redundant set of corn mRNAs in GenBank, we
estimate that there are about 50,000 different protein coding genes in Zea.
All of the sequence data from this study have been submitted to DDBJ/GenBank/EMBL
under accession numbers EU940701-EU977132 (FLI cDNA) and FK944382-FL482108 (EST).
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Atanassov, I., Russinova, E., Antonov, L.,
Atanassov, A., 1998. Expression of an anther-specfic chalcone synthase-like
gene is correlated with uninucleate microspore development in Nicotina
sylvestris. Plant Molecular Biology 38, 1169-1178.
Two cDNA clones, specifically expressed in Nicotiana sylvestris
in anthers during uninucleate microspore development, were isolated using a
subtractive hybridization approach. Sequence analysis showed that one of
them, NSCHSLK, displayed a high level of similarity to several
anther-specific chalcone synthase-like (CHSLK) proteins and an ORF from
chromosome 1 of Arabidopsis thaliana. A lower, but significant,
similarity to chalcone synthases and closely related enzymes (CHSRE) was
also detected. The structure of the nschslk gene was found to be
typical of the chalcone (chs)/stilbene (sts) synthase family. Expression of
NSCHSLK mRNA was confined to microspores and tapetal cells. UV- irradiation
or infection with Phytophthora parasitica var. nicotianae of
transgenic Nicotiana benthamiana plants carrying a chimeric nschslk/GUS
gene indicated that the nschslk promoter exhibits the same anther-specific,
developmentally regulated expression pattern. Comparison of CHSRE and CHSLK
polypeptide sequences revealed some important similarities and differences
between the two groups. The data presented in this study, suggest that the
anther-specific chslk genes represent a separate sub-family of plant
polyketide synthases related to chs/sts in terms of gene structure,
polypeptide sequence and the possible catalytic mechanism, but differing in
substrate/product specificity. The putative role of CHSLK enzymes in anther
development and particularly in exine synthesis is discussed.
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Burbulis, I. E., Iacobucci, M., Shirley, B. W.,
1996. A null mutation in the first enzyme of flavonoid biosynthesis does not
affect male fertility in Arabidopsis. Plant Cell 8, 1013-1025.
Flavonoids are a major class of secondary metabolites that serves
a multitude of functions in higher plants, including a recently discovered
role in male fertility. Surprisingly, Arabidopsis plants deficient in
flavonoid biosynthesis appear to be fully fertile. Using RNA gel blot
analysis and polymerase chain reaction-based assays, we have shown that a
mutation at the 3' splice acceptor site in the Arabidopsis chalcone synthase
gene completely disrupts synthesis of the active form of the enzyme. We also
confirmed that this enzyme, which catalyzes the first step of flavonoid
biosynthesis, is encoded by a single-copy gene. HPLC analysis of whole
flowers and stamens was used to show that plants homozygous for the splice
site mutation are completely devoid of flavonoids, This work provides
compelling evidence that despite the high levels of these compounds in the
pollen of most plant species, flavonoids are not universally required for
fertility. The role of flavonoids in plant reproduction may therefore offer
an example of convergent functional evolution in secondary metabolism.
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Feinbaum, R. L., Ausubel, F. M., 1988.
Transcriptional regulation of the Arabidopsis thaliana chalcone
synthase gene. Molecular and Cellular Biology 8, 1985-1992.
We have cloned an Arabidopsis thaliana chalcone
synthase (CHS) gene on the basis of cross-hybridization with a
Petroselinum hortense CHS cDNA clone. The protein sequence deduced from
the A. thaliana CHS DNA sequence is at least 85% homologous to the
CHS sequences from P. hortense, Antirrhinum majus, and
Petunia hybrida. Southern blot analysis indicated that CHS is a
single-copy gene in A. thaliana. High-intensity light treatment of
A. thaliana plants for 24 h caused a 50-fold increase in CHS enzyme
activity and an accumulation of visibly detectable levels of anthocyanin
pigments in the vegetative structures of these plants. A corresponding
increase in the steady-state level of CHS mRNA was detected after
high-intensity light treatment for the same period of time. The accumulation
of CHS mRNA in response to high-intensity light was due, at least in part,
to an increased rate of transcription of the CHS gene as demonstrated by
nuclear runoff experiments.
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Hihara, Y., Hara, C., Uchimiya, H., 1996. Isolation
and characterization of two cDNA clones for mRNAs that are abundantly
expressed in immature anthers of rice (Oryza sativa L.). Plant
Molecular Biology 30, 1181-1193.
The relationship between the length of anthers and the stage of
development of microspores was examined in rice (Oryza sativa L. cv.
Hayayuki). Anthers of = 2 mm and 2.1-2.2 mm in length and those ready to
dehiscence were determined to be at the uninucleate, binucleate and
trinucleate microspore stage, respectively. Two cDNAs (YY1 and YY2),
representing genes that are specifically expressed in anthers at the
uninucleate microspore stage, were isolated and characterized. YY1 cDNA
encoded an open reading frame of 95 amino acids. Eight cysteine residues
with the potential to form disulfide bridges were present in the amino acid
sequence. There was a hydrophobic region at the N-terminus of the putative
protein, suggesting that the YY1 protein might be secreted. This cysteine
motif and the hydrophobic N-terminus are conserved among products of several
anther-specific genes or cDNAs isolated from various plant species. These
proteins are thought to form a superfamily of proteins that are confined to
anthers. The YY1 transcript was localized in the tapetal cells and the
peripheral cells of the vascular bundle. YY2 cDNA encoded an open reading
frame of 389 amino acids and the deduced amino acid sequence exhibited
substantial homology to that of chalcone synthase. Expression of YY2 mRNA
was confined to the tapetal cells. The genes correspond to YY1 and YY2 cDNAs
were shown to exist as single copies in the rice genome.
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Jaillon, O., Aury, J. M., Noel, B., Policriti, A.,
Clepet, C., Casagrande, A., Choisne, N., Aubourg, S., Vitulo, N., Jubin, C.,
Vezzi, A., Legeai, F., Hugueney, P., Dasilva, C., Horner, D., Mica, E.,
Jublot, D., Poulain, J., Bruyere, C., Billault, A., Segurens, B., Gouyvenoux,
M., Ugarte, E., Cattonaro, F., Anthouard, V., Vico, V., Del, F. C., Alaux,
M., Di Gaspero, G., Dumas, V., Felice, N., Paillard, S., Juman, I., Moroldo,
M., Scalabrin, S., Canaguier, A., Le Clainche, I., Malacrida, G., Durand,
E., Pesole, G., Laucou, V., Chatelet, P., Merdinoglu, D., Delledonne, M.,
Pezzotti, M., Lecharny, A., Scarpelli, C., Artiguenave, F., Pe, M. E.,
Valle, G., Morgante, M., Caboche, M., Adam-Blondon, A. F., Weissenbach, J.,
Quetier, F., Wincker, P., 2007. The grapevine genome sequence suggests
ancestral hexaploidization in major angiosperm phyla. Nature 449, 463-467.
The analysis of the first plant genomes provided unexpected
evidence for genome duplication events in species that had previously been
considered as true diploids on the basis of their genetics. These
polyploidization events may have had important consequences in plant
evolution, in particular for species radiation and adaptation and for the
modulation of functional capacities. Here we report a high-quality draft of
the genome sequence of grapevine (Vitis vinifera) obtained from a
highly homozygous genotype. The draft sequence of the grapevine genome is
the fourth one produced so far for flowering plants, the second for a woody
species and the first for a fruit crop (cultivated for both fruit and
beverage). Grapevine was selected because of its important place in the
cultural heritage of humanity beginning during the Neolithic period. Several
large expansions of gene families with roles in aromatic features are
observed. The grapevine genome has not undergone recent genome duplication,
thus enabling the discovery of ancestral traits and features of the genetic
organization of flowering plants. This analysis reveals the contribution of
three ancestral genomes to the grapevine haploid content. This ancestral
arrangement is common to many dicotyledonous plants but is absent from the
genome of rice, which is a monocotyledon. Furthermore, we explain the
chronology of previously described whole-genome duplication events in the
evolution of flowering plants.
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Tuskan, G. A., Difazio, S., Jansson, S., Bohlmann,
J., Grigoriev, I., Hellsten, U., Putnam, N., Ralph, S., Rombauts, S.,
Salamov, A., Schein, J., Sterck, L., Aerts, A., Bhalerao, R. R., Bhalerao,
R. P., Blaudez, D., Boerjan, W., Brun, A., Brunner, A., Busov, V., Campbell,
M., Carlson, J., Chalot, M., Chapman, J., Chen, G. L., Cooper, D., Coutinho,
P. M., Couturier, J., Covert, S., Cronk, Q., Cunningham, R., Davis, J.,
Degroeve, S., Dejardin, A., Depamphilis, C., Detter, J., Dirks, B., Dubchak,
I., Duplessis, S., Ehlting, J., Ellis, B., Gendler, K., Goodstein, D.,
Gribskov, M., Grimwood, J., Groover, A., Gunter, L., Hamberger, B., Heinze,
B., Helariutta, Y., Henrissat, B., Holligan, D., Holt, R., Huang, W., Islam-Faridi,
N., Jones, S., Jones-Rhoades, M., Jorgensen, R., Joshi, C., Kangasjarvi, J.,
Karlsson, J., Kelleher, C., Kirkpatrick, R., Kirst, M., Kohler, A., Kalluri,
U., Larimer, F., Leebens-Mack, J., Leple, J. C., Locascio, P., Lou, Y.,
Lucas, S., Martin, F., Montanini, B., Napoli, C., Nelson, D. R., Nelson, C.,
Nieminen, K., Nilsson, O., Pereda, V., Peter, G., Philippe, R., Pilate, G.,
Poliakov, A., Razumovskaya, J., Richardson, P., Rinaldi, C., Ritland, K.,
Rouze, P., Ryaboy, D., Schmutz, J., Schrader, J., Segerman, B., Shin, H.,
Siddiqui, A., Sterky, F., Terry, A., Tsai, C. J., Uberbacher, E., Unneberg,
P., Vahala, J., Wall, K., Wessler, S., Yang, G., Yin, T., Douglas, C., Marra,
M., Sandberg, G., Van de Peer, Y., Rokhsar, D., 2006. The genome of black
cottonwood, Populus trichocarpa (Torr. & Gray). Science 313,
1596-1604.
We report the draft genome of the black cottonwood tree, Populus
trichocarpa. Integration of shotgun sequence assembly with genetic
mapping enabled chromosome-scale reconstruction of the genome. More than
45,000 putative protein-coding genes were identified. Analysis of the
assembled genome revealed a whole-genome duplication event; about 8000 pairs
of duplicated genes from that event survived in the Populus genome. A
second, older duplication event is indistinguishably coincident with the
divergence of the Populus and Arabidopsis lineages. Nucleotide substitution,
tandem gene duplication, and gross chromosomal rearrangement appear to
proceed substantially more slowly in Populus than in Arabidopsis. Populus
has more protein-coding genes than Arabidopsis, ranging on average from 1.4
to 1.6 putative Populus homologs for each Arabidopsis gene. However, the
relative frequency of protein domains in the two genomes is similar.
Overrepresented exceptions in Populus include genes associated with
lignocellulosic wall biosynthesis, meristem development, disease resistance,
and metabolite transport.
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Walden, A. R., Walter, C., Gardner,
R. C., 1999.
Genes
expressed in Pinus radiata male cones include homologs to
anther-specific and pathogenesis response genes. Plant Physiology 121,
1103-1116.
We describe the isolation and characterization of 13 cDNA clones
that are differentially expressed in male cones of Pinus radiata (D. Don).
The transcripts of the 13 genes are expressed at different times between
meiosis and microspore mitosis, timing that corresponds to a burst in
tapetal activity in the developing anthers. In situ hybridization showed
that four of the genes are expressed in the tapetum, while a fifth is
expressed in tetrads during a brief developmental window. Six of the seven
cDNAs identified in database searches have striking similarity to genes
expressed in angiosperm anthers. Seven cDNAs are homologs of defense and
pathogen response genes. The cDNAs identified are predicted to encode a
chalcone-synthase-like protein, a thaumatin-like protein, a serine hydrolase
thought to be a putative regulator of programmed cell death, two
lipid-transfer proteins, and two homologs of the anther-specific A9 genes
from Brassica napus and Arab_idopsis. Overall, our results support the
hypothesis that many of the reproductive processes in the angiosperms and
gymnosperms were inherited from a common ancestor.
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Wu, S., O'Leary, S. J. B., Gleddie, S., Eudes, F.,
Laroche, A., Robert, L. S., 2008. A chalcone synthase-like gene is highly
expressed in the tapetum of both wheat (Triticum aestivum L.) and
triticale (x Triticosecale Wittmack). Plant Cell Reports 27,
1441-1449.
A novel anther-specific chalcone synthase-like gene, TaCHSL1, was
isolated and characterized. The TaCHSL1 transcript was detected only within
the tapetum during the "free" and early vacuolated microspore stages in both
wheat and triticale. Sequence analysis indicated that the 41.8 kDa TaCHSL1
deduced protein belongs to a small distinct group of type III polyketide
synthases that includes angiosperm and gymnosperm orthologs shown to be
anther-specific. TaCHSL1 sequence characteristics and conservation, as well
as its restricted expression pattern, point to a distinct and important
biochemical role in developing anthers.
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Tropf, S., Kärcher, B., Schröder,
G., Schröder, J., 1995. Reaction mechanisms of homodimeric plant
polyketide synthases (stilbene and chalcone synthase): a single active site
for the condensing reaction is sufficient for synthesis of stilbenes,
chalcones, and 6'-deoxychalcones. Journal of Biological Chemistry 270,
7922-7928.
Stilbene (STS) and chalcone (CHS) synthases are homodimeric, related
plant-specific polyketide synthases. Both perform a sequential condensation
of three acetate units to a starter residue to form a tetraketide
intermediate that is folded to the ring systems specific to the different
products. Protein cross-linking and site-directed mutagenesis identified a
subunit contact site in position 158, close to the active site (Cys-169).
This suggested that the active site pockets may be neighboring, possibly
alternating in the condensing reactions rather than acting independently.
This was investigated by coexpression of active site mutants with
differently mutated, inactive proteins. With both STS and CHS, the
heterodimers synthesized the end products, indicating that each subunit
performed all three condensations. In co-action with a monomeric reductase,
CHS also synthesizes 6'-deoxychalcone, but with the chalcone as second
product when using plant preparations. The two enzymes expressed as single
species in E. coli synthesized both products, and both were also
obtained with a CHS heterodimer containing a single active site. The results
showed that 6'-deoxychalcone synthesis required no other plant factor and
that the formation of two products may be an intrinsic property of the
interaction between dimeric CHS and monomeric reductase.
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