Relationship between DNA cycle and growth rate in Synechococcus sp. strain PCC 6301

Flow cytometry was used to examine cell cycle regulation in Synechococcus sp. strain PCC 6301 under a variety of growth conditions. The DNA frequency distributions of exponentially growing and dark-blocked populations confirmed that this cyanobacterium contains multiple chromosome copies even at very slow growth rates. Furthermore, the presence of major peaks corresponding to other than 2" chromosome copies strongly suggests that DNA replication is initiated asynchronously. Although this suggestion is at odds with the standard formulation of the procaryotic cell cycle model, it is similar to recent observations of asynchrony in Escherichia coli replication mutants.