The role of the primary-like sigma factor SigC was studied in Synechocystis. Under high temperature stress (48 degrees C) the Delta sigC inactivation strain showed a lower survival rate than the control strain. The Delta sigC strain grew poorly at 43 degrees C in liquid cultures under normal air. However, change to 3% CO(2) enhanced growth of Delta sigC at 43 degrees C. Differences in expression of many genes related to the carbon concentrating mechanisms between the control and the Delta sigC strain were recorded with a genome-wide DNA microarray. We suggest that low solubility Of CO(2) at high temperature is one of the factors contributing to the poor thermotolerance of the Delta sigC strain. (C) 2007 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.