PROMOTER RECOGNITION BY A CYANOBACTERIAL RNA-POLYMERASE - IN-VITRO STUDIES WITH THE CALOTHRIX SP. PCC-7601 TRANSCRIPTIONAL FACTORS RCAA ANDRCAD

To study the transcriptional apparatus and the mechanisms that control gene expression in cyanobacteria, the RNA polymerase was purified fro m the filamentous Calothrix sp. PCC 7601 and used in in vitro transcri ption assays. Conditions required for specific transcription initiatio n to occur were analyzed with the eleven Calothrix PCC 7601 genes for which the 5' ends have been mapped. Most of the transcripts directly o btained did not have the expected size, providing a test for looking a t specific transcription factors. Addition of RcaA, a protein that bin ds to the promoter region of the phycobiliprotein cpeBA operon, restor ed accurate initiation of transcription in the in vitro system for thr ee phycobiliprotein promoters. RcaA thus is a transcription factor tha t allows to mimick in vivo transcription. In parallel, the functional properties of the Escherichia coil and cyanobacterial RNA polymerases were compared. The enteric enzyme could not precisely initiate transcr iption at the promoter of a phycobiliprotein gene and, reciprocally, t he cyanobacterial RNA polymerase could initiate transcription at P-lac UV5, but not from wild-type PI,, promoters. The different behaviours o f the enzymes are discussed in the light of the structural differences that exist between subunits of the RNA polymerases.