Transcription in chloroplasts
The Chloroplast genome differs from the nuclear genome in having a much smaller information content (10-3 to 10-4 fold less), but with 100-10,000 copies per cell, making upto 15% of the cellular DNA. There are about 120 plastid genes in the chloroplast genome, about 60 coding for plastid proteins, 30 coding for a part of photosynthetic apparatus (photosystem II or PSII, Cytochrome b6/f, photosystem I or PSI, ATP synthase and RUBISCO), and the remaining genes with unknown functions. (Many subunits of photosynthetic apparatus are coded by nuclear genes). It has been shown that in Chlamydomonas reinhardtii, most chloroplast genes are transcribed independently of each other as monocistronic transcripts, but chloroplast genomes of maize and other higher plants contain several polycistronic transcription units, a single unit encoding subunits of different photosynthetic complexes.
The mechanism of transcription for majority of chloroplasts genes resembles that in prokaryotes in several respects including the following : (i) Promoter sequences are found at -10 and -35 positions for all kinds of genes (rRNAs, tRNAs and mRNAs), so that same RNA polymerase is used for the transcription of these three classes of genes, (ii) Termination of transcription in plastid genes is facilitated by the formation of hairpin structures (like in prokaryotes) due to the presence of short inverted repeat sequences, (iii) Chloroplast mRNAs have neither a cap at the 5' end, nor a long poly A tail at their 3' end (as in prokaryotes).