chloroplasten genom12
TRANSCRIPT
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Chloroplast genome:Evolution structureand regulation of thegene expression
Darin I Peshev
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orop as s
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Chloroplasts
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Development of
chloroplasts
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Origin and evolution of the
chloroplasts
The cyanobacterialgenome contains morethan 3000 potential
protein genes Present-day chloroplast
genome contains onlyabout 75 protein genes.
Nucleus encoded,
proteins are highlysimilar to those incyanobacterium
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Chloroplast genome
evolution
Rapid and massivereduction in number ofgenes:
Transferred to nucleus Lost
80-90% of plastid proteinsare encoded in nucleus
Great overlap in genecontent suggests that lastcommon ancestor ofcpDNA had ~300 genes
Synechocystis
3573 kbp~3000 genes
Porphyrachloroplast201 kbp
~250 genes
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1) 45 genes present in
all genomes
2) Unique losses (68)
outnumbered by
parallel losses
(122)3) Confirms that
ancestral plastid
genome was
already highly
reduced from thatof cyanobacteria
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Chloroplast genome (The
plastome)
The plastid chromosome exists as a negativelysupercoiled molecule
Plastome, is a circular double-stranded molecule of120 to 180 kilobase pairs (kbp).
Each plastid contains tens to hundreds of copies of themolecule, organized into several nucleoids that molecules are present as monomers, dimers, trimers and
tetramers in a relative amount of 1, 1/3, 1/9 and 1/27
Chromosome organization is highly conserved
Two inverted repeat (IR) regions separating a largeand a small single copy (LSC and SSC, respectively)region
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The inverted repeat (IR)
Ranges from 5bk to 76kb in
length
IR contains rRNA genes plus
others:
None in brown algae(5kb)
10 in tobacco (25kb)
40 in geranium (76kb)
Present in: Land plants (exc.
legumes)
Chlorophytes
Chromophytes
Partial in conifers
Pinus
Nicotiana
Porphyra
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Coding regions
4 ribosomal RNA
genes;
30 tRNA genes;
More than 72 genes
encoding
polypeptides;
Several conserved
reading frames (ycf)coding for proteins of
yet unknown function;
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The plastid genes coding for
polypeptides can be classified
into several categories:1) Genes coding for the prokaryotic RNA
polymerase core-enzyme;
2) Genes coding for proteins of thetranslational apparatus;
3) For the photosynthetic apparatus
4) Genes encoding subunits of the NADHdehydrogenase(ndh).
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Plastid gene propertiesNo plastid tRNA gene codes for its 3'-
CCA end;
no RNA, even of small size, is importedinto chloroplasts.
plastid genes of higher plants contain
single intronsintrons have been classified into two
groups, group I and II
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Chloroplast division
Many of the Bryophytesand Pteridophytespossess one single plastid per cell
Isoetes, an evolved fern, possesses onechloroplast per meristematic cell andseveral chloroplasts in mature cells
cell. In angiosperms, the dark-greenspinach leaves contain more than 200chloroplasts per mesophyll celland
Arabidopsis contains more than 100plastids per mesophyll cell
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Genetic basis forchloroplast division Arabidopsis of arc (accumulation and
replication of chloroplasts) mutants Inverse relationship exists between the
number of chloroplasts and size.
Correlates the total surface ofchloroplasts to cell size.
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Plastid division control
When the number of plastids per cell is low, plastiddivision is probably controlled by the cell cycle
When cells contain a large number of plastids, they donot divide synchronously
The regulatory pathway that determines when a plastidenters the division cycle is also unknown
Also, it has been recently discovered that division ofplastids, besides the overall control by the cell, hasconserved prokaryotic-like mechanisms.
In bacteria FtsZ, minC, minD and minE genes
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Replication of plastid DNA
All the plastid chromosomes (about
10,000) in a cultured cell of tobacco
replicate in one cell cycle DNA synthesis occurs outside of
compact nucleoids (68 kDa DNA
compacting nucleoid protein inhibits DNAsynthesis in vitro)
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Mechanisms governing thereplicationof plastid DNA
Formation of two
displacement loops (D-
loops)
Cairns-type of replicativeintermediate
The plastid DNA
contains also a rolling
circle replicativeintermediate
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Enzymes participating in
the replication-DNA polymerase (resistant to aphidicolin;
inhibited by ethidium bromide; smaller)
43 kDa protein
120 kDa primase
Two different topoisomerases I
Topoisomerase II activities have been
detected in chloroplasts of higher plantsGyrase activity
DNA helicase of 78 kDa
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Transcriptional apparatus
Regulation of chloroplast gene expression
occurs at several levels:
1) Several RNA polymerases (PEP and NEP)
2) sigma-like transcription initiation factors are
controlling the activity of the plastid encoded
plastid RNA polymerase (PEP)
3) Transcription factors can interact with the two
types of RNA polymerase and thus regulate the
choice of the transcriptional system
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Plastid-encoded plastid
RNA polymerases (PEP)
Similar to the RNApolymerase subunitsof cyanobacteria
Plastid encodedrpoA, rpoB, rpoC1and rpoC2 genescorrespond really to
polypeptides presentin a highly purifiedRNA polymerase
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Transcriptioninitiation factors of the sigma-70
type Translated products of six different cDNAs show strong
similarities with the prokaryotic sigma 70 factors Three of them, it has been shown that they are transported into
chloroplasts;
Light-induced regulation of gene expression couldinvolve phosphorylation dephosphorylation ofsigma-like factors;
N-terminal parts of the plant factors have differentfunctions than the N-terminal part of the sigma-70
factor from E. coli. The C-terminal part which is responsible for the DNA
promoter recognition, is functionally conservedbetween prokaryotes and plastids.
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SIG1 most prokaryotic like, recognizes allessential E. coli promoters, It recognizesspecifically the plant prokaryotic-type rbcL
promoter SIG2 recognizes specifically the
lessconserved prokaryotic-type P1 promoter ofthe rrn operon encoding the rRNA species
SIG3
recognizes all plastid prokaryoticpromoters that have been analysed (function ofSIG3 is less specific).
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AAG box AGF factor binds for it,
necessary for the transcription of the
bluelight activatedpsbD operon in barley CDF2 binds specifically to the promoter
region of the rrn operon and regulates
expression of rRNA in plastids.
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Nuclear-encoded plastidRNA polymerases NEP Resembling the bacteriophage T7 RNA polymerase(
110 kDa monomeric RNA polymerase, recognizing aT7 promoter)
NEP transcribes the genes encoding elements of thegenetic system, rather than the photosynthesis genes
Hypothesis attributes specific functions to NEP inhousekeeping gene expression during early phases ofplant and plastid development, and to PEP inphotosynthesis-related gene expression in later phases
of plant and plastid development A second NEP:
NEP2 is recruited to the PC promoter by the CDF2 factor
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Thank you foryour time