FtsH (filamentous temperature sensitive) genes are members of the AAA (ATPases associated with diverse cellular activities) class of proteases and contain an essential AAA domain, which contains the ATP binding site, as well as a conserved zinc binding domain. FtsH was first identified in E. coli where the enzyme functions both as an ATP-dependent protease and as a chaperone. Many phenotypes are conferred by fish mutations, and FtsH is thought to have both soluble and membrane-bound substrates. FtsH genes comprise a small gene family in cyanobacteria and higher plants. Its role in photosynthetic organisms is less clear than in E. coli, but it has been implicated in the assembly of components of the thylakoid membrane and in the Dl repair cycle of Photosystem II, during which photodamaged Dl proteins are degraded and replaced by new copies. There are four FtsH genes in the unicellular cyanobacterium, Synechocystis sp. PCC 6803. This work focuses on understanding the functional role of these genes. An insertion mutant was previously obtained for one of the genes (slrO228), and in this study mutants were obtained for the other three genes. Two of the mutants were not studied further because of incomplete segregation of wild type genome copies; these mutants probably define essential genes. One of the mutants had no discernible phenotype, suggesting it is not essential for growth. Finally, detailed analyses of slr0228 showed that it is photosensitive and has reduced amounts of Photosystem I. This suggests that slr0228 is involved in photoprotection.