The transcriptional apparatus of chloroplasts

Abstract The transcription of genes encoded by the chloroplast genome relies on the activity of a chloroplast‐specific, DNA‐dependent RNA polymerase. This activity was characterized in its soluble and in its DNA‐bound form. The plastome genes rpoA, rpoB, rpoC1, and rpoC2 were identified as coding for the subunits, α, ß, ß, and ß”;, respectively, of the chloroplast RNA polymerase by sequence similarity of the encoded peptides with their bacterial counterparts. Expression of the rpo genes was confirmed by detection of the corresponding subunits in highly purified preparations from maize chloroplasts and by Western blotting of crude extracts with antisera raised against individual subunits. Transcription initiation is governed by sigma‐like factors (probably not encoded in the plastome) and by promoters that resemble those associated with prokaryotic genes. Recent data concerning actively transcribed genes within plastomes that have suffered a complete loss of RNA polymerase genes highlight the possibility of additional nuclear‐encoded forms of this essential enzyme.