The Synechocystis sp. PCC 6803 genome encodes four putative homologues of the AAA protease FtsH, two of which (slr0228 and sll1463) have been subjected to insertional mutagenesis in this study. Disruption of sll1463 had no discernible effect but disruption of slr0228 caused a 60% reduction in the abundance of functional photosystem I, without affecting the cellular content of photosystem II or phycobilisomes. Fluorescence and immunoblotting analyses show reductions in PS I polypeptides and possible structural alterations in the residual PS I, indicating an important role for slr0228 in PS I biogenesis. ß

A mutant of the cyanobacterium Synechococcus sp. strain PCC 7942 carrying a disrupted gene encoding glucose-6-phosphate dehydrogenase (zwf) produced no detectable glucose-6-phosphate dehydrogenase as assessed by enzyme assay and Western blot (immunoblot) analysis. This mutant exhibited significantly impaired dark viability.

A genetic characterization of nine strains of Proeh]orococcus originating from various depths of the Mediterranean Sea, the Sargasso Sea, the North and the tropical Atlantic Ocean and the Pacific Ocean was performed by restriction fragment length polymorphism (RFLP) mapping using probes for rbcL, rbcS, psbA and woxA. This study revealed extensive genetic variation among strains, which were grouped into two distinct clusters. Unexpectedly, with one exception (TATLI strain), the strains clustered by isolation depth (i.e. near surface versus deep isolates) rather than by geographic origin. These relationships were confirmed by secondary RFLP analysis of psbA amplification products using Proehloroeoccus-specific po[ymerase chain reaction (PCR) primers.