rpbA controls transcription of the constitutive phycocyanin gene set in Fremyella diplosiphon

Three gene sets encode alpha and beta subunits of the phycobiliprotein phycocyanin (PC) in the filamentous cyanobacterium Fremyella diplosiphon. The cpcB1A1 set (encodes PC1) is constitutively expressed, whereas the cpcB2A2 set (encodes PC2) is expressed only in red light and the cpcB3A3 set (encodes PC3 is expressed only during sulfur-limited growth. Primary pigment mutant strain FdBM1 is characterized by elevated levels of PC. DNA hybridization analysis shelved that like many pigment mutants in our strain collection, strain FdBM1 harbors an extra genomic copy of endogenous transposon Tn5469. By direct cloning from FdBM1 genomic DNA, the extra copy of Tn5469 was localized to an open reading frame, which we have designated the rpbA gene. Complementation experiments correlated rpbA activity to the phenotype of strain FdBM1, The predicted RpbA protein contains two regions resembling the characterized helix-turn-helix motif which is involved in DNA recognition by many bacterial and phage transcription regulator proteins. RNA hybridization analysis showed that relative to the parental strain Fd33, the level of transcripts from cpcB1A1, but not cpcB2A2 or cpcB3A3, was significantly elevated in strain FdBM1. Introduction of the intact rpbA gene into strain FdBM1 restored the cpcB1A1 transcript level to that of strain Fd33. These results suggest that the rpbA gene product functions in controlling constitutive transcription from the cpcB1A1 gene set, possibly as a DNA-binding transcriptional repressor element.