Summary: The gene encoding the principal σ factor from sp. strain PCC 7002 was isolated and characterized. The sp. strain PCC 7002 gene encodes a protein of 375 amino acids (43.7 kDa) that is required for viability under normal growth conditions. The SigA protein was overproduced in and the purified protein was used to raise polyclonal antiserum in rabbits. This antiserum was used in immunoblot analyses of partially purified RNA polymerase from sp. strain PR6000. The probable translational start site was identified by a comparison of amino acid sequencing results obtained with SigA proteins overproduced in with immunoblot analyses of SigA protein in crude preparations of RNA polymerase from the cyanobacterium. The gene is encoded on a transcript of 1700 bases that initiates 496 nucleotides upstream from the probable translational start site. The abundance of transcripts decreases rapidly after the removal of combined nitrogen from the growth medium.


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