REGULATION OF RDNA TRANSCRIPTION IN CHLOROPLASTS - PROMOTER EXCLUSIONBY CONSTITUTIVE REPRESSION

Spinach chloroplasts contain two types of RNA polymerases. One is mult imeric and Escherichia coli-like. The other one is not E. coli-like an d might represent a monomeric enzyme of 110 kD. The quantitative relat ion of the two polymerases changes during plant development. This rais es the question, how are plastid genes transcribed that contain E. col i-like and non-E. coli-like promoter elements during developmental pha ses when both enzymes are present? Transcription of the spinach plasti d rrn operon promoter is initiated at three sites: P1, PC, and P2. P1 and P2 are preceded by E. coli-like promoter elements that are recogni zed by E. coli RNA polymerase in vitro. However, in vivo, transcriptio n starts exclusively at PC. We analyzed different promoter constructio ns using in vitro transcription and gel mobility-shift studies to unde rstand why P1 and P2 are not used in vivo. Our results suggest that th e sequence-specific DNA-binding factor CDF2 functions as a repressor f or transcription initiation of the E. coli-like enzyme at P1 and P2. W e propose a mechanism of constitutive repression to keep the rm operon in all developmental phases under the transcriptional control of the non E. coli-like RNA polymerase.