The gene encoding the principal σ factor from Synechococcus sp. strain PCC 7002 was isolated and characterized. The Synechococcus sp. strain PCC 7002 sigA 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 Escherichia coil 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 Synechococcus sp. strain PR6000. The probable in vivo translational start site was identified by a comparison of amino acid sequencing results obtained with SigA proteins overproduced in E. coli with immunoblot analyses of SigA protein in crude preparations of RNA polymerase from the cyanobacterium. The sigA gene is encoded on a transcript of 1700 bases that initiates 496 nucleotides upstream from the probable in vivo translational start site. The abundance of sigA transcripts decreases rapidly after the removal of combined nitrogen from the growth medium.
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