The and genes, encoding two alternative sigma factors of the unicellular marine cyanobacterium sp. PCC 7002, were cloned and characterized. Strains in which the and genes were insertionally inactivated were viable under standard laboratory conditions, indicating that SigB and SigC are group 2 sigma factors. Starvation for either nitrogen or carbon caused an increase in mRNA levels. Transcripts for the gene initially increased but then decreased during nitrogen and carbon starvation. The SigC protein could not be identified in cyanobacterial extracts using antisera to sp. PCC 7002 SigA or RpoD from The ratio of the principal vegetative sigma factor, SigA, to SigB decreased during either nitrogen starvation or carbon starvation, and the levels of SigB also increased in the mutant strain. These results imply that SigB and SigC play roles in modifying transcription in response to changes in carbon and nitrogen availability in this cyanobacterium.


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