Gene expression in chloroplast
Transcription:
- The chloroplast genome differs from the nuclear genome in having very less genes, making 15% of cellular DNA.
- There are about 120 plastid genes in chloroplast genome, about 60 genes coding for plastid proteins, 30 coding for a part of photosynthetic apparatus( ps-II, cyt b6lf, ps-I, ATP synthetase) and remaining genes with unknown functions.
- For expression of 120 genes of chloroplast, factors coded by nuclear DNA are required, so that more than 1000 nuclear genes required for the expression of mt DNA(genes).
- The mechanism of transcription in chloroplast resembles that in prokaryotes such as i) promoter sequences are found at -10 and -35 positions for all kinds of RNAs, hence one RNAP is used for the transcription.
- Termination of transcription in plastid genes is facilitated by the formation of hair pin structures due to the presence of short inverted repeat sequences.
- Chloroplast mRNA have neither a cap at the 5' end nor a poly A tail at their 3'end. However three classes of introns are found in chloroplast genes which need to be removed out during RNA processing.
Translation:
- The translation process in chloroplast takes place by using 70s ribosomes. tRNAs and other factors specific to organelle.
- Initiation of translation takes place by formyl methionyl tRNA in chloroplast although no formylation takes place in cytoplasm of eukaryotes.
- Translation in chloroplast is inhibited by chloromophenicol where as in cytoplasm is inhibited by cyclohexamide since 70s and 80s ribosomes are sensitive with chloromophenicol and cyclohexamide respectively. Thus protein synthesis can be studied either only in cytoplasm or in chloroplast.
- The studies regarding chlorophyll binding protein synthesis is made. In absence of light the mRNA's of chlorophyll binding protein and chloroplast protein remains associated with thalakoid bound polysomes but are not translated. The transfer of seedling to sunlight induces synthesis of proteins although no increase in transcription is observed, hence translation is light induced process.
- The plastid ribosomes directly bind to initiator codon for initiation of translation as in prokaryotes and many factors required for translation are the products of nuclear genes.
- Protein synthesis in isolated chloroplasts of pea was studied. The proteins produced in isolated chloroplast are large sub unit of fraction-I protein, five unidentified proteins of internal lamellar system and two or three unidentified proteins of envelope. Only few of large number of proteins produced by chloroplast genome and remaining produced by nuclear genome.
- It is found that large sub-unit of Fraction-I protein is synthesized under the influence of chloroplast DNA. The small sub-unit is synthesized in the cytoplasm under the influence of nuclear DNA. This small sub-unit pass into the chloroplast and associated with large sub-unit to form Fraction-I protein.
