CELL-CYCLE REGULATION IN MARINE SYNECHOCOCCUS SP STRAINS

The cell cycle behavior of four marine strains of the unicellular cyan obacterium Synechococcus sp. was analyzed by examining the DNA frequen cy distributions of exponentially growing and dark-blocked populations and by considering the patterns of change in these distributions duri ng growth under a diel light-dark cycle. The two modes of cell cycle r egulation previously identified in a freshwater and coastal marine Syn echococcus isolate, respectively, were represented among the three ope n-ocean strains we examined. The first of these modes of regulation is consistent with the slow-growth case of the widely accepted prokaryot ic cell cycle paradigm. The second appears to involve asynchronous ini tiation of chromosome replication, the presence of multiple chromosome copies at low growth rates, and variability in chromosome copy number among cells in the population. These characteristics suggest the invo lvement of a large probabilistic component in cell cycle regulation wh ich could make the application of cell cycle-based estimators of in si tu growth rate to Synechococcus populations problematic.