Specific recognition of the cyanobacterial psbA promoter by RNA polymerases containing principal sigma factors.

The psbA2 gene of a unicellular cyanobacterium, Microcystis aeruginosa K-81, encodes a D1 protein homolog in the reaction center of photosynthetic Photosystem II. To clarify the promoter recognition by a sigma factor of RNA polymerase, in vivo and in vitro analyses were performed for the photosynthetic gene. Although the specific transcript from the psbA2 promoter, whose sequence is of Escherichia coli consensus type, was observed in both cyanobacterium K-81 and E. coli cells, the expression was light-dependent in K-81 whereas it was constitutive in E. coli under the conditions of light and darkness (L/D). The specific psbA2-dependent transcripts were also detected in vitro by RNA polymerases containing the principal sigma factors, E. coli sigma70 and K-81 sigmaA1 (constitutively exists in K-81 grown under L/D cycles). Furthermore, a series of promoter fragments were constructed to confirm minimal cis elements for the in vitro psbA2 transcription. A -80 to +6 or -38 to +46 region, the sequences of which consisted of a core promoter (-38 to +6), was identified as the potential minimal cis element using the RNA polymerase fraction (*EsigmaA1) containing sigmaA1 partially purified from K-81. These results suggest that the psbA2 transcription with the minimal sequence was induced by the RNA polymerase (EsigmaA1) containing the principal sigma factor, sigmaA1, under both light and dark conditions in K-81.