Salinity-regulated replication of the endogenous plasmid pSY10 from the marine cyanobacterium Synechococcus sp.

The endogenous plasmid pSY10 in the marine cyanobacterium Synechococcus sp. NKBG042902 is maintained at a high copy number when cells are grown in seawater and at a low copy number when cultured in freshwater. The mechanism of salinity-regulated replication of this plasmid was investigated. Transcription of repA was depressed under freshwater, which was accompanied by a low copy number of pSY10 and the appearance of a new protein that was expressed only in cells cultured in freshwater. This protein was observed to bind to putative repA promoters (Prep1 and Prep2) on pSY10. Moreover, this protein was observed only in Synechococcus sp. NKBG042902. The data suggest that this protein(s) regulates repA transcription in pSY10, stress responsive and encoded by the host chromosome.