Chloroplasten genom12

8/12/2019 Chloroplasten genom12

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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

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Chloroplasten genom12