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.
Further, it has also been shown in C. reinhardtii, that for the expression of each of the nearly 120 chloroplast genes, factors coded by 5-70 nuclear loci are required, so that more than 1000 nuclear genes (5-10% of the nuclear genome) are involved in the expression of these genes. These nuclear encoded factors may function at the level of transcription, RNA processing, RNA maturation, translation, or during the assembly of photosynthetic complexes. While differential transcription has been documented for several chloroplast genes, regulation is manly exercised at the post transcriptional level.