The goals of this proposal were two-fold: (1) to analyze the impact of mutations in the psbO gene (coding for the Mn-stabilizing protein, MSP) on O{sub 2}-evolution; and (2) to analyze membrane assembly in cyanobacteria, especially the effect of iron deficiency on this process. The mutations in the psbO gene were performed in the transformable and photoheterotrophic cyanobacterium Synechocystis sp. PCC6803, whereas the research with membrane assembly and iron deficiency was performed in the transformable strain Synechococcus sp. PCC7942. Our work with the Synechocystis psbO gene demonstrated that the MSP protein is not an essential protein but serves a regulatory function. We produced a deletion mutant, which we call {triangle}psbO. The {triangle}psbO strain was still capable of photosynthetic growth and evolved O{sub 2} at rates of 1/3 to 1/2 that of the wild type. We have characterized this strain in some detail and have reported the results in two publications in Biochemistry. The second of the these two papers was published during this grant period and is enclosed.