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(Last
modification: 10. May 2010)
'Orphan' PKS in Ipomoea
Der Genus
Ipomoeae (Wikipedia: english,
deutsch) ist der grösste in der Familie der
Convolvulaceae,
mit mehr als 500 Mitgliedern. Viele sind berühmt für ihre schönen Blütenfarben, aber
in manchen Fällen werden sie auch als Unkraut angesehen, z.B. in der
Landwirtschaft. Interessante Beispiele für Ipomoea-Arten sind:
-
Ipomoea
purpurea,
die 'Common Morning Glory', see:
Ipomoea purpurea, deutsch: Purpur-Prunkwinde (anscheinend bisher keine
extra deutsche Seite in Wikipedia)
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Ipomoea nil,
eine eng verwandte Species in Japan, die 'Japanese Morning Glory',
siehe:
Ipomoea nil
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Ipomoea
tricolor,
e.g. das Cultivar 'Heavenly Blue', benannt nach der wundervollen
blauen Farbe der Blüten, Wikipedia; english:
Ipomoea tricolor, deutsch:
Ipomoea tricolor.
Die Struktur des blauen Pigmentes ('Heavenly
Blue Anthocyanin') ist seit langem bekannt; es ist ein
Paeonidin-Derivat mit einem sehr komplexen Substitutionsmuster:
Mehr...
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Ipomoea
batatas
(Sweet Potato, Süßkartoffel); dies scheint die einzige wichtige Nutzpflanze
in dieser Familie zu sein, eine sehr wichtige in mehreren Ländern; siehe
Wikipedia, englisch:
Sweet Potato, deutsch:
Süßkartoffel
Link zu einigen
Photos
und der Struktur des 'Heavenly
Blue Anthocyanin'
Typ
III PKS in Ipomoea
In
einer ersten Publikation wurden 13 CHS-Typ Sequenzen von
I. purpurea
und sechs anderen Ipomoea Arten analysiert. Vier Gene wurden identifiziert:
CHS-A, CHS-B, CHS-C, und ein Pseudogen, CHS-PS (Durbin et al., 1995).
Alle gehörten eindeutig in die Familie der CHS-verwandten Proteine, aber hatten
nicht wirklich eine enge Verwandtschaft zu funktionell eindeutig definierten
CHS; die Bezeichnung als CHS hatte keine solide Grundlage.
Etwas später wurden zwei neue Sequenzen aus der 'Japanese Morning Glory' (Ipomoea
nil, eine eng verwandte Art) charakterisiert; sie wurden CHS-D and CHS-E
genannt
(Fukada-Tanaka et al., 1997;
Habu et al., 1997). Diese sahen
schon eher aus wie typische CHS, und genetische Experimente zeigten, dass CHS-D
das wichtigste Gen für die Pigmente in den Blüten ist, während CHS-E
hauptsächlich in den schwach gefärbten Blütenstielen exprimiert wird (Fukada-Tanaka et al., 1997;
Habu et al., 1998;
Johzuka-Hisatomi
et al., 1999; Durbin et al., 2000;
Coberly and Rausher, 2003).
Die detaillierte Analyse all dieser Sequenzen führte zu einigen interessanten
Einsichten in die Evolution der Genfamilie in diesen Arten (Glover et al.,
1996; Huttley et al., 1997; Rausher et al., 1999;
Durbin et al., 2000;
Hoshino et al., 2001;
Durbin et al., 2001;
Clegg and Durbin, 2003),
und in die möglichen Konsequenzen von 'pathway
degeneration' (Zufall and Rausher, 2004).
Ein Teil dieser Arbeiten sollte vielleicht in seinen Interpretationen neu
gesehen werden, da nicht für alle Artikel bekannt war, dass die zuerst
entdeckten Gene sicherlich nicht für CHS kodierten (s. unten).
Funktionelle Analyse
Die publizierten Daten beziehen sich auf CHS-D and CHS-E; die
Enzymaktivitäten wurden nach Expression rekombinanter Proteine in E. coli
untersucht (Shiokawa
et al., 2000). In beiden Fällen waren die Hauptprodukte
Naringeninchalcon und CTAL (das Pyron-Nebenprodukt nach drei Kondensationen).
Dies ist typisch für CHS in vitro, und die Identifizierung als CHS
passt zu den genetischen Daten.
'Orphan
PKS' in Ipomoeae
CHS-A
und
CHS-B wurden in der gleichen Arbeitsgruppe untersucht, aber die Ergebnisse
wurden nicht publiziert. Prof. H. Noguchi (School
of Pharmaceutical Sciences, University of Shizuoka, Yada, Shizuoka 422-8526,
Japan) schickte mir jedoch im Dezember 2004 in einer persönlichen Mitteilung
eine Zusammenfassung der Ergebnisse, mit der Erlaubnis, die Daten zu zitieren:
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CHS-A ist eine 'Orphan PKS':
Es synthetisierte nicht Naringeninchalcon aus 4-Coumaroyl-CoA, sondern
Bisnoryangonin; d.h. es führte nur zwei Kondensationen durch.
Ganz anders war das Ergebnis mit einem um eine -CH2 Gruppe
verkürzten Starter, Phenylacetyl-CoA: Das Produkt war
2,4,6-Trihydroxyphenylbenzylketone; dies war das Produkt erwartet von
drei Kondensationen gefolgt von einer CHS-Typ Ringfaltung.
Interessanterweise kann dies eine 'richtige' CHS auch, wie mit der CHS aus Scutellaria baicalensis
(Labiatae) bereits früher gezeigt (Morita et al., 2000).
Und es kommt noch mehr Unerwartetes: Mit Acetyl-CoA oder Malonyl-CoA
erhielt man Dihydroxy-2-methylchromone, genau wie mit der
Pentaketid-Chromon-Synthase
aus Aloe arborescens (Abe et
al., 2005). Dies bedeutete, dass das Enzym mit so kleinen Startern
vier Kondensationsreaktionen durchführen konnte, gefolgt von
einer CHS-Typ Ringfaltung. Diese Ergebnisse zeigten, dass das Protein keine
CHS ist. In den Pflanzen gab es keine Naturstoffe, die sich aus den
identifizierten Reaktions-Produkten ableiten lassen, und deshalb sollte CHS-A
erstmal als Orphan PKS angesehen werden.
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CHS-B ist eine 'Orphan PKS':
Dieses Protein wurde mit den gleichen Substraten getestet. Die Produkte
waren in allen Fällen die Pyrone nach zwei
Kondensationsreaktionen, und so kann auch dies keine CHS sein. CHS-B ist
eine Orphan PKS mit der gleichen Begründung wie CHS-A.
Links zu anderen Beispielen von 'Orphan PKS'
Zum Seitenanfang
Zur Übersicht
über 'Orphan PKS'
Zitate
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Abe, I.,
Utsumi, Y., Oguro, S., Morita, H., Sano, Y., Noguchi, H., 2005. A plant
type III polyketide synthase that produces pentaketide chromone. Journal of
the American Chemical Society 127, 1362-1363.
Here we report a
novel plant-specific type III PKS that catalyzes formation of a
pentaketide chromone, 5,7-dihydroxy-2-methylchromone, from five
molecules of malonyl-CoA (Scheme 1B). Remarkably, replacement of a
single amino acid residue Met207 (corresponding to the Medicago
sativa CHS active site residue Thr197) yielded a mutant enzyme that
efficiently produces aromatic octaketides, SEK4 and SEK4b, the products
of the minimal PKS for the benzoisochromanequinone actinorhodin (act
from Streptomyces coelicolor) (Scheme 1C). A cDNA encoding
the pentaketide chromone synthase (PCS) (the GenBankTM accession no.
AY823626) was cloned and sequenced from young roots of aloe (Aloe
arborescens), a medicinal plant rich in aromatic polyketides
including chromones and anthraquinones, by RT-PCR using degenerate
primers based on the conserved sequences of known CHSs as described
before. A 1,212-bp open reading frame encoded a Mr 44,568 protein
with 403 amino acids. The deduced amino acid sequence showed 50-60%
identity to those of CHS-superfamily enzymes from other plants; 58%
identity (232/403) with M. sativa CHS, and 50% identity (206/403)
with R. palmatum ALS that catalyzes formation of a heptaketide,
aloesone (2-acetonyl-7-hydroxy-5-methylchromone), from acetyl-CoA and
six molecules of malonyl-CoA. A. arborescens PCS maintains almost
identical CoA binding site and the catalytic triad of Cys164, His303,
and Asn336 (numbering in M. sativa CHS) absolutely conserved in
all type III PKSs. Furthermore, most of the active site residues
including Met137, Gly211, Gly216, Pro375, along with Phe215 and Phe265,1
are well conserved in PCS (Fig. 1). The CHS-based homology modeling
predicted that A. arborescens PCS has the same threedimensional
overall fold as M. sativa CHS5a, with a cavity volume (1124 Å3)
slightly larger than that of CHS (1019 Å3), and almost as large as that
of R. palmatum ALS (1173 Å3).
Zurück
Go to
pentaketide chromone
synthase
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Clegg, M.
T., Durbin, M. L., 2003. Tracing floral adaptations from ecology to
molecules. Nat. Rev. Genet. 4, 206-215.
Flowers have long
fascinated humans. The scientific study of floral biology unifies many
diverse areas of research, ranging from systematics to ecology, and from
genetics to molecular biology. Despite this unity, few plant species
offer the experimental versatility to encompass all levels of biological
investigation in a single system. An exception is the morning glory
genus Ipomoea, in which a broad picture of floral evolution, ranging
from ecology to molecular biology, is emerging.
Zurück
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Coberly,
L. C., Rausher, M. D., 2003. Analysis of a chalcone synthase mutant in
Ipomoea purpurea reveals a novel function for flavonoids:
amelioration of heat stress. Molecular Ecology 12, 1113-1124.
Flavonoids are
thought to function in the plant stress response and male fertility in
some, but not all, species. We examined the effects of a self-fertile
chalcone synthase null allele, a, for the effects of heat and
light stress on fertilization success and flower production in
Ipomoea purpurea. Pollen recipients and pollen donors of both
homozygous genotypes exhibit reduced fertilization success at high
temperatures, indicating that high temperature acts as a stress-lowering
fertilization success. Homozygous aa individuals exhibit reduced
male and female fertilization success, compared to AA individuals,
at high temperatures but not at low temperatures. In addition, aa
individuals produce fewer flowers than AA individuals at low
temperatures, but not at high temperatures. These results suggest that
flavonoids alleviate heat stress on fertilization success. They also
suggest that pleiotropic effects at the A locus may explain the
low frequency of the a allele in natural populations.
Zurück
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Durbin, M.
L., Denton, A. L., Clegg, M. T., 2001. Dynamics of mobile element activity
in chalcone synthase loci in the common morning glory (Ipomoea purpurea).
Proceedings of the National Academy of Sciences of the United States of
America 98, 5084-5089.
Mobile element
dynamics in seven alleles of the chalcone synthase D locus (CHS-D) of
the common morning glory (Ipomoea purpurea) are analyzed in the
context of synonymous nucleotide sequence distances for CHS-D exons. By
using a nucleotide sequence of CHS-D from the sister species Ipomoea
nil (Japanese morning glory [Johzuka-Hisatomi, Y., Hoshino, A., Mori,
T., Habu, Y. & Iida, S. (1999) Genes Genet. Syst. 74, 141-147],
it is also possible to determine the relative frequency of insertion and
loss of elements within the CHS-D locus between these two species. At
least four different types of transposable elements exist upstream of
the coding region, or within the single intron of the CHS-D locus in
I. purpurea. There are three distinct families of miniature
inverted-repeat transposable elements (MITES), and some recent
transpositions of Activator/Dissociation (Ac/Ds)-like elements (Tip100),
of some short interspersed repetitive elements (SINEs), and of an
insertion sequence (InsIpCHSD) found in the neighborhood of this locus.
The data provide no compelling evidence of the transposition of the
mites since the separation of I. nil and I. purpurea
roughly 8 million years ago. Finally, it is shown that the number and
frequency of mobile elements are highly heterogeneous among different
duplicate CHS loci, suggesting that the dynamics observed at CHS-D are
locus-specific.
Zurück
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Durbin, M.
L., Learn, G. H., Huttley, G. A., Clegg, M. T., 1995. Evolution of the
chalcone synthase family in the genus Ipomoea. Proceedings of the
National Academy of Sciences of the United States of America 92, 3338-3342.
The evolution of the
chalcone synthase [CHS; malonyl-CoA:4-coumaroyl-CoA malonyltransferase (cyclizing),
EC 2.3.1.74] multigene family in the genus Ipomoea is explored. Thirteen
CHS genes from seven Ipomoea species (family Convolvulaceae) were
sequenced-three from genomic clones and the remainder from PCR
amplification with primers designed from the 5' flanking region and the
end of the 3' coding region of Ipomoea purpurea Roth. Analysis of
the data indicates a duplication of CHS that predates the divergence of
the Ipomoea species in this study. The Ipomoea CHS genes are among the
most rapidly evolving of the CHS genes sequenced to date. The CHS genes
in this study are most closely related to the Petunia CHS-B gene, which
is also rapidly evolving and highly divergent from the rest of the
Petunia CHS sequences.
Zurück
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Durbin, M.
L., McCaig, B., Clegg, M. T., 2000. Molecular evolution of the chalcone
synthase multigene family in the morning glory genome. Plant Molecular
Biology 42, 79-92.
Plant genomes appear
to exploit the process of gene duplication as a primary means of
acquiring biochemical and developmental flexibility. Thus, for example,
most of the enzymatic components of plant secondary metabolism are
encoded by small families of genes that originated through duplication
over evolutionary time. The dynamics of gene family evolution are well
illustrated by the genes that encode chalcone synthase (CHS), the first
committed step in flavonoid biosynthesis. We review pertinent facts
about CHS evolution in flowering plants with special reference to the
morning glory genus, Ipomoea. Our review shows that new CHS genes are
recruited recurrently in flowering plant evolution. Rates of nucleotide
substitution are frequently accelerated in new duplicate genes, and
there is clear evidence for repeated shifts in enzymatic function among
duplicate copies of CHS genes. In addition, we present new data on
expression patterns of CHS genes as a function of tissue and
developmental stage in the common morning glory (I. purpurea). These
data show extensive differentiation in gene expression among duplicate
copies of CHS genes. We also show that a single mutation which blocks
anthocyanin biosynthesis in the floral limb is correlated with a loss of
expression of one of the six duplicate CHS genes present in the morning
glory genome. This suggests that different duplicate copies of CHS have
acquired specialized functional roles over the course of evolution. We
conclude that recurrent gene duplication and subsequent differentiation
is a major adaptive strategy in plant genome evolution.
Zurück
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Fukada-Tanaka, S., Hoshino, A., Hisatomi, Y., Habu, Y., Hasebe, M., Iida,
S., 1997. Identification of new chalcone synthase genes for flower
pigmentation in the Japanese and common morning glories. Plant and Cell
Physiology 38, 754-758.
New cDNA sequences for
chalcone synthase (CHS), a key enzyme in the flavonoid biosynthesis,
were obtained from the Japanese and common morning glories; they are
more closely related to other CHS sequences than the six previously
described CHS genes from the same plants. The newly isolated CHS-D gene
is abundantly expressed in the pigmented flower buds, while its
expression is drastically reduced in the white flower buds. Thus CHS-D
appears to produce major CHS transcripts for flower pigmentation.
Zurück
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Glover, D.
E., Durbin, M. L., Huttley, G. A., Clegg, M. T., 1996. Genetic diversity in
the common morning glory. Plant Species Biology 11, 41-50.
Populations of the
common morning glory in the southeastern US are characterized by a
striking diversity of flower color polymorphisms. This diversity is
probably a consequence of horticultural escapes from cultivation in the
18-th and 19-th centuries. More than 15 years of research in our
laboratory has shown that some color phenotypes are selected by virtue
of their differential attraction to insect pollinators. We have studied
genetic diversity at isozyme and ribosomal DNA loci and we find reduced
diversity in the southeastern US compared to Mexican populations. In an
effort to link ecological genetics to molecular evolution, we have
cloned and characterized the chalcone synthase (CHS) gene family in
morning glory and we have studied the expression of CHS genes in flower
development. We have also initiated an investigation of spatial patterns
of diversity at CHS genes by sampling and sequencing genes from US and
Mexican populations. These investigations reveal (1) that the four CHS
genes (CHS A, B, C, and PS) characterized to date evolve rapidly in
morning glory and that the gene family in Ipomoea is of relatively
recent origin (approximately 21 million years); (2) the duplicate genes
in Ipomoea group into two categories (CHS A, C versus CHS B, PS) that
may indicate a functional divergence between chalcone synthase and
stilbene synthase activities; (3) levels of molecular diversity for CHS
A genes sampled from Mexico are much higher than observed in US
collections suggesting a major population bottleneck associated with the
introduction of morning glory into the southeastern US; and (4) the
ratio of amino acid substitution to synonymous substitution between
Ipomoea species is remarkably high (about 5.4 synonymous to amino acid
substitutions) compared to CHS genes in other plant species. Taken
together these data portray a rapidly evolving gene family where
functional divergence may arise repeatedly in evolution, despite the
central role of chalcone synthase in flavonoid metabolism.
Zurück
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Habu, Y.,
Fukada-Tanaka, S., Hisatomi, Y., Iida, S., 1997. Amplified restriction
fragment length polymorphism-based mRNA fingerprinting using a single
restriction enzyme that recognizes a 4-bp sequence. Biochemical and
Biophysical Research Communications 234, 516-521.
Using amplified
restriction fragment length polymorphism (AFLP) technology, we have
developed a new protocol for the fingerprinting of mRNA that allows
systematic comparison of the differential expression of genes between
mRNA samples. The major advantage of our protocol is the use of only a
single restriction enzyme that recognizes a 4-bp sequence but allows
screening of large numbers of different cDNAs. Using this new protocol,
we compared mRNA samples obtained from the flower buds of two lines of
the common morning glory (Ipomoea purpurea) with red and white flowers,
respectively. Approximately 50 bands were observed in each lane of a
denaturing polyacrylamide gel and the results were highly reproducible,
as indicated by the results of analysis of two sets of independent mRNA
samples. Two cDNA fragments, which were differentially amplified in the
samples from the two lines, were shown to have been derived from a
single gene that was actively expressed in the buds of red flowers but
not in those of white flowers. A full-length cDNA of this gene was
cloned from a bud cDNA library. Sequence analysis showed that this cDNA
carries a sequence highly homologous to the chalcone synthase (CHS)
genes, the key enzyme in the flavonoid biosynthetic pathway.
Zurück
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Habu, Y.,
Hisatomi, Y., Iida, S., 1998. Molecular characterization of the mutable
flaked allele for flower variegation in the common morning glory. Plant
Journal 16, 371-376.
The mutable flaked (or
a(f)) lines of the common morning glory bear white flowers with colored
flakes and sectors. The a(f) allele shows incomplete dominance. Plants
in the heterozygous state A/a(f) bear lightly colored flowers with
intensely colored flakes and occasionally with white sectors. We showed
that the mutable a(f) allele is caused by insertion of a new
transposable element, Tip100, into the CHS-D gene intron. Tip100 is 3.9
kb long and belongs to the Ac/Ds family. Although the timing and
frequency of the flower variegation vary in different lines, they carry
an identical mutable allele. We also noticed that a flaked subline,
bearing variegated flowers, carries a Tip100 derivative, Tip100-1 The
structure of Tip100-1 contains an additional 48 bp terminal sequence as
tandem repeats and its integration site is identical to that of Tip100.
Another line, with stable white flowers, is a double mutant carrying two
copies of Tip100 in the CHS-D gene. These results are discussed with
regard to the variegated phenotypes of flowers in various mutable lines.
Zurück
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Hoshino,
A., Johzuka-Hisatomi, Y., Iida, S., 2001. Gene duplication and mobile
genetic elements in the morning glories. Gene 265, 1-10.
We review gene
duplication and subsequent structural and functional divergence in the
anthocyanin biosynthesis genes in the Japanese and common morning
glories and discuss their evolutionary implications. These plants appear
to contain at least six copies of the CHS gene and three tandem copies
of the DFR gene. Of these, the CHS-D and DFR-B genes are mainly
responsible for flower pigmentation and mutations in these genes confer
white flowers. We compared the genomic sequences of these duplicated
genes between the two morning glories and found small mobile
element-like sequences (MELSs) and direct repeats (DRs) in introns and
intergenic regions. The results indicate that the MELS elements: and DRs
play significant roles in divergence after gene duplication. We also
discuss DNA rearrangements occurring before and after speciation of
these morning glories. DNA transposable elements belonging to the Ac/Ds
or En/Spm families have acted as major spontaneous mutagens in these
morning glories. We also describe the structural features of the first
Mlr-related element found in the morning glories and polymorphisms found
in the same species.
Zurück
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Huttley,
G. A., Durbin, M. L., Glover, D. E., Clegg, M. T., 1997. Nucleotide
polymorphism in the chalcone synthase-A locus and evolution of the chalcone
synthase multigene family of common morning glory (Ipomoea
purpurea). Molecular Ecology 6, 549-558.
Chalcone synthase (CHS)
is a small multigene family with at least four members (CHS-A, B, C and
PS) in common morning glory Ipomoea purpurea ROTH. The chalcone
synthase enzyme performs the initial condensation reaction that results
in the 15-carbon three- ring structure that is the backbone of flavonoid
biosynthesis. The biochemical pathway that commences with CHS is
important in plant disease defence, pigment biosynthesis and UV
protection. Accordingly, it is of substantial interest to characterize
levels and patterns of molecular diversity for genes that encode this
important enzyme. We report the sequence of 19 CHS-A alleles from
Mexican and American populations of common morning glory. American
populations of this annual self-compatible vine are believed to have
been introduced from Mexico, where the species is native. Individual
plants were sampled from populations of common morning glory throughout
Mexico and the southeastern USA. Four American alleles were sequenced
and these, together with one allele from Mexico City, were identical in
primary nucleotide sequence. These data suggest a restricted origin for
the American population, probably as a consequence of selection for
domestication by pre-Columbian peoples. Additionally the Mitontic (Chiapas,
Mexico) population is significantly more homogeneous than expected by
chance indicating that this population may also have experienced a
recent population bottleneck. Estimates of nucleotide diversity from the
Mexican CHS-A alleles were high. We present evidence that these
estimates may, in part, result from low to moderate levels of interlocus
recombination/gene conversion. We also present evidence that the ancient
duplication of the CHS gene family, preceding the origin of the genus
Ipomoea, was associated with heterogeneity in the rate of substitution
between the resulting gene family members. The group of gene family
members whose sequences possess a signature amino acid of the closely
related Stilbene synthase exhibit a significantly faster proportional
rate of nonsynonymous substitution.
Zurück
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Johzuka-Hisatomi, Y., Hoshino, A., Mori, T., Habu, Y., Iida, S., 1999.
Characterization of the chalcone synthase genes expressed in flowers of the
common and Japanese morning glories. Genes & Genetic Systems 74, 141-147.
The CHS genes
encoding chalcone synthase for flavonoid biosynthesis in the common and
Japanese morning glories comprise a multigene family. Among these
Ipomoea CHS genes, the CHS-D gene is the most abundantly expressed in
the pigmented young flower buds and is primarily responsible for flower
pigmentation. Majority of the remaining CHS transcripts in the flower
buds are produced from the CHS-E gene. We characterized the genomic DNA
segments of these CHS-D and CHS-E genes. Both genes have two exons with
identical intron positions and carry several copies of two mobile
element-like sequences with short terminal inverted repeats, MELS3 and
MELS6 of around 200-300 bp. Small tandem repeats were also found in
these CHS gene regions. The CHS-D and CHS-E genes are expressed
predominantly in flower limbs and tubes, respectively. These structural
and functional features and their evolutionary implications are
discussed.
Zurück
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Morita,
H., Takahashi, Y., Noguchi, H., Abe, I., 2000. Enzymatic formation of
unnatural aromatic polyketides by chalcone synthase. Biochemical and
Biophysical Research Communications 279, 190-195.
Substrate
specificity of recombinant chalcone synthase (CHS) from Scutellaria
baicalensis (Labiatae) was investigated using chemically synthesized
aromatic and aliphatic CoA esters. It was demonstrated for the first
time that CHS converted benzoyl-CoA to phlorobenzophenone
(2,4,6-trihydroxybenzophenone) along with pyrone by-products. On the
other hand, phenylacetyl-CoA was enzymatically converted to an unnatural
aromatic polyketide, phlorobenzylketone
(2,4,6-trihydroxyphenylbenzylketone), whose structure was finally
confirmed by chemical synthesis. Furthermore, in agreement with earlier
reports, S. baicalensis CHS also accepted aliphatic CoA esters,
isovaleryl-CoA and isobutyryl-CoA, to produce phloroacylphenones. In
contrast, hexanoyl-CoA only afforded pyrone derivatives without
formation of a new aromatic ring. It was noteworthy that both aromatic
and aliphatic CoA esters were accepted in the active site of the enzyme
as a starter substrate for the complex condensation reaction. The low
substrate specificity of CHS thus provided further insight into the
structure and function of the enzyme.
Zurück
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Rausher,
M. D., Miller, R. E., Tiffin, P., 1999. Patterns of evolutionary rate
variation among genes of the anthocyanin biosynthetic pathway. Molecular
Biology and Evolution 16, 266-274.
The anthocyanin
biosynthetic pathway is responsible for the production of anthocyanin
pigments in plant tissues and shares a number of enzymes with other
biochemical pathways. The sir core structural genes of this pathway have
been cloned and characterized in two taxonomically diverse plant species
(maize and snapdragon). We have recently cloned these genes for a third
species, the common morning glory, Ipomoea purpurea. This
additional information provides an opportunity to examine patterns of
evolution among genes within a single biochemical pathway. We report
here that upstream genes in the anthocyanin pathway have evolved
substantially more slowly than downstream genes and suggest that this
difference in evolutionary rates may be explained by upstream genes
being more constrained because they participate in several different
biochemical pathways. In addition, regulatory genes associated with the
anthocyanin pathway tend to evolve more rapidly than the structural
genes they regulate, suggesting that adaptive evolution of flower color
may be mediated more by regulatory than by structural genes. Finally,
for individual anthocyanin genes, we found an absence of rate
heterogeneity among three major angiosperm lineages. This rate constancy
contrasts with an accelerated rate of evolution of three CHS-like genes
in the Ipomoea lineage, indicating that these three genes have diverged
without coordinated adjustment by other pathway genes.
Zurück
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Shiokawa,
K.-I., Inagaki, Y., Morita, H., Hsu, T.-J., Iida, S., Noguchi, H., 2000.
The functional expression of the CHS-D and CHS-E genes of the common
morning glory (Ipomoea purpurea) in Escherichia coli and
characterization of their gene products. Plant Biotechnology
17,
203-210.
The genes encoding
chalcone synthase (CHS) in the common morning glory (Ipomoea purpurea)
comprise a multigene family, and they are divided into two subfamilies.
The genes in a subfamily including the CHS-A, CHS-B and CHS-C genes are
distantly related to the other known CHS sequences in a phylogenetic
tree, whereas the CHS- D and CHS- E genes in another subfamily are more
closely related to the well - characterized CHS genes. As an initial
step to elucidate biological function of these CHS genes in I.
purpurea, the CHS-D and CHS-E cDNAS were expressed in Escherichia
coli with different expression systems. The recombinant CHS-D and
CHS-E proteins both showed CHS activity to produce naringenin
chalcone.These results are discussed with regard to the biological roles
of the CHS-D and CHS-E genes in flower pigmentation in I. purpurea.
We have also discussed these CHS-D and CHS-E enzyme as members of plant
specific polyketide synthases.
Zurück
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Zufall, R.
A., Rausher, M. D., 2004. Genetic changes associated with floral adaptation
restrict future evolutionary potential. Nature 428, 847-850.
A commonly accepted
evolutionary principle is that adaptive change constrains the potential
directions of future evolutionary change. One manifestation of this is
Dollo's law, which states that character elimination is irreversible.
Although the common occurrence of irreversibility has been documented by
phylogenetic analyses of phenotypic transitions, little is known about
the underlying causes of this phenomenon. One explanation for
evolutionary irreversibility relies on the fact that many
characteristics result from interactions between multiple gene products.
Such characteristics may often be eliminated by inactivation of just one
gene in the network. If they serve no other functions, other genes of
the network are then free to accumulate mutations or evolve new
functions. Evolutionary change after character loss results in the
accumulation of redundant loss-of-function mutations. Such pathway
degeneration makes it very unlikely that the characteristic will
re-evolve, because multiple simultaneous mutations would be required.
Here we describe what appear to be the initial stages of such
degeneration in the anthyocyanin pigment pathway associated with an
adaptive change from blue to red flowers in the morning glory Ipomoea
quamoclit.
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