Transcription of Plastid Genes
Transcription of plastid genes resembles transcription in prokaryotes in many respects. In fact, when E. coli is transformed with the plastid genes rbcL and psbA, transcription is initiated in it at a site similar or identical to the one used by the chloroplast RNA polymerase.

This result is consistent with studies showing that the DNA sequence elements which direct transcription initiation of plastid genes (TTGaca and TAtaaT, located 35 and 10 nucleotides upstream of the site of transcription initiation, respectively) are similar to prokaryotic promoter elements.

Furthermore, E. coli RNA polymerase transcribes atpB more efficiently than rbcL, whereas the reverse is true for the chloroplast enzyme. Finally, transcription in E. coli is sensitive to rifampicin, whereas chloroplast transcription is not. These results show that transcription initiation in E. coli and chloroplasts have some overall similarities but differ significantly in specific ways.

It is possible that differences in E. coli and chloroplast transcription result from differences in RNA polymerase composition. The E. coli tran­scription apparatus consists of an RNA polymerase containing four subunits (β, β’, α, σ) and accessory factors (nusA, nusB, rho, tau). Chloroplast RNA polymerase preparations contain from 5 to 14 subunits.

First, it has been reported that all subunits of an isolated RNA polymerase preparation are synthesized on cytoplasmic ribosome.The fact that genes encoding α, β and β ' subunits are localized in plastids raised the possibility that some RNA polymerase subunits may be encoded by both nuclear and plastid genes.

Second, transcriptionally active complexes of DNA and RNA polymerase have been isolated which show preferential transcription of rRNA. Based on the properties of these preparations it has been proposed that rRNA and protein genes may be transcribed by different RNA polymerases.