Regulation of plastid rDNA transcription by interaction of CDF2 with two different RNA polymerases

The plastid genome is known to be transcribed by a plastid-encoded prokaryo tic-type RNA polymerase (PEP) and by a nucleus-encoded phage-type RNA polym erase (NEP). The spinach plastid rrn operon promoter region harbours three different, overlapping promoters. Two of them are of the prokaryotic type. The third promoter is a non-consensus-type NEP promoter, We separated three different transcriptional activities from spinach chloroplasts: PEP, the p hage-type RNA polymerase NEP-1, and a third, hitherto undescribed transcrip tional activity (NEP-2), NEP-2 specifically transcribes the rrn operon in t he presence of the transcription factor CDF2, CDF2 was previously shown to recruit PEP to the rrn promoter to repress transcription. Together, our res ults suggest the existence of a third RNA polymerase in plastids and a mech anism of rDNA transcriptional regulation that is based on the interaction o f the transcription factor CDF2 with two different transcriptional systems.