Expression of two alternative sigma factors of Synechococcus sp. strain PCC 7002 is modulated by carbon and nitrogen stress

Department of Biochemistry and Molecular Biology and Center for Biomolecular Structure and Function, The Pennsylvania State University, University Park, PA 16802, USA Author for correspondence: Donald A. Bryant. Tel: + 1 814 865 1992. Fax: + 1 814 863 7024. e-mail: dab14@psu.edu ABSTRACT The sigB and sigC genes, encoding two alternative sigma factors of the unicellular marine cyanobacterium Synechococcus sp. PCC 7002, were cloned and characterized. Strains in which the sigB and sigC 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 sigB mRNA levels. Transcripts for the sigC gene initially increased but then decreased during nitrogen and carbon starvation. The SigC protein could not be identified in cyanobacterial extracts using antisera to Synechococcus sp. PCC 7002 SigA or RpoD from Bacillus subtilis. 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 sigC 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. Keywords: RNA polymerase, sigma factors, transcription, cyanobacteria, nutrient stress Present address: Department of Biology, State University of New York -College at Fredonia, Fredonia, NY 14063, USA.