ISOLATION AND CHARACTERIZATION OF THE GENE ENCODING THE PRINCIPAL SIGMA FACTOR OF THE VEGETATIVE CELL RNA-POLYMERASE FROM THE CYANOBACTERIUM ANABAENA SP STRAIN PCC-7120

The filamentous cyanobacterium Anabaena sp. strain PCC 7120 responds to combined nitrogen deprivation by forming specialized nitrogen-fixing cells at regular intervals along the filament. Genetic and biochemical studies have indicated that regulation of gene expression during differentiation occurs at the transcriptional level. As part of a characterization of RNA polymerase during differentiation, the gene encoding the 52-kDa principal sigma factor of the Anabaena sp. strain PCC 7120 vegetative-cell RNA polymerase was isolated by using an oligonucleotide probe based on the sequence of the N-terminal seven amino acids of the purified protein. sigA codes for a 390-amino-acid polypeptide that has a predicted molecular weight of 45,641. The amino acid sequence of the polypeptide encoded by sigA contains four regions corresponding to conserved domains of the principal RNA polymerase sigma factors of Escherichia coli (sigma-70) and Bacillus subtilis (sigma-43). Thus, although the subunit composition of cyanobacterial RNA polymerase core differs from that of other eubacteria (G. J. Schneider and R. Haselkorn, J. Bacteriol. 170:4136-4140, 1988), the principal sigma factor of at least one cyanobacterium is typically eubacterial. In contrast to sigma-70 and sigma-43 operon organization, sigA is monocistronic and encodes two transcripts of 1.7 and 2.2 kb. The abundance of the 1.7-kb transcript remains constant under both nitrogen-replete and nitrogen-limiting conditions, whereas the 2.2-kb transcript is induced following the removal of combined nitrogen. Continued or enhanced transcription of sigA under nitrogen starvation conditions is consistent with the observation that the principal RNA polymerase in differentiating cells contains SigA.