A rapidly labeled photosynthetic membrane protein was identified in the cyanobacterium Synechococcus PCC7942 R2 as the 32 kDA protein that is involved in electron transport and quinone binding in the photosystem II complex. Partial proteolysis of the membrane-bound protein indicates that the internal architecture and the topology of the Synechococcus 32 kDa protein resembles the analogous protein of higher plants. In addition to the R2 wild-type strain, we characterized three psbA-inactivated Synechococcus strains, in which two of the three endogenous psbA genes were inactivated. In all strains, a 32 kDa protein cross-reacts with an antiserum that was raised against a higher-plant 32 kDa protein and displays in vivo light-dependent turnover. In Synechococcus, the herbicide DCMU inhibits the 32 kDa protein turnover at similar concentration ranges as in higher plants; however, a fraction of the molecules always displays a DCMU-insensitive degradation.