Identification of multiple RNA polymerase sigma factor homologs in the cyanobacterium Anabaena sp. strain PCC 7120: cloning, expression, and inactivation of the sigB and sigC genes.

The sigA gene of Anabaena sp. strain PCC 7120, encoding the principal RNA polymerase sigma factor, and the complement of the rpoD oligonucleotide (K. Tanaka, T. Shiina, and H. Takahashi, Science 242:1040-1042, 1988) were used as probes to isolate two genes, sigB and sigC, which encode two putative sigma factors exhibiting high degrees of similarity to SigA, to HrdA, -B, -C, and -D of Streptomyces coelicolor, and to KatF of Escherichia coli. sigB and sigC code for polypeptides of 332 and 416 amino acids with predicted molecular weights of 38,431 and 47,459, respectively. sigB and sigC mRNAs are detectable only under nitrogen-limiting conditions. Insertional inactivation of sigB and sigC indicates that neither gene alone is essential for nitrogen fixation or heterocyst differentiation.