Two Anabaena sp. strain PCC 7120 DNA-binding factors interact with vegetative cell- and heterocyst-specific genes

The DNA-binding factor BifA (previously called VF1) binds upstream of the developmentally regulated site-specific recombinase gene xisA in the cyanobacterium Anabaena sp. strain PCC 7120. Besides binding xisA, BifA also binds the glnA, rbcL, and nifH promoter regions. DNase I footprint analysis of BifA binding to glnA showed a protected region -125 to -148 bp upstream of the translation start site. The binding site is between the major glnA transcription start site used in vegetative cells (RNAII) and the major transcription start site used under nitrogen-deficient conditions (RNAI). The two BifA-binding sites on the rbcL promoter were localized to a 24-bp region from +12 to -12 nucleotides and to a 12-bp region from -43 to -54 nucleotides with respect to the transcription start site. Comparison of the BifA binding sites on the glnA, xisA, and rbcL upstream regions revealed the consensus recognition sequence TGT(N9 or 10) ACA. We have identified a second DNA-binding activity (factor 2) that interacts with rbcL and xisA upstream regions. Factor 2 can be resolved from BifA by heparin-Sepharose chromatography and was present in a bifA mutant. Analysis of partially purified vegetative cell and heterocyst extracts showed that whereas BifA was present in both cell types, factor 2 was present only in vegetative cells. DNase I footprint analysis of factor 2 binding to rbcL showed protection of a 63-bp region between positions -15 and -77 with respect to the transcription start site. The factor 2 binding site on xisA was localized to a 68-bp region that showed considerable overlap with the BifA binding sites.