The a factors of RNA polymerase play central roles when bacteria adapt to different environmental conditions. We studied heat-shock responses in the cyanobacterium Synechocystis sp. PCC6803 using the 47 factor inactivation strains Delta sigB, Delta sigD and Delta sigBD. The SigB factor was found to be important for short-term heat-shock responses and acquired thermotolerance. The normal high-temperature induction of the hspA gene depended on the SigB factor. The SigD a factor had a role in high-temperature responses as well, and the double inactivation strain Delta sigBD grew more slowly at 43 degrees C than the Delta sigB and Delta sigD strains. (c) 2005 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.