Cell cycle regulation and the timing of chromosome replication in a marine Synechococcus (cyanobacteria) during light- and nitrogen-limited growth

Cell cycle behavior in the marine Synechococcus strain WH8101 was examined in detail over a wide range of light- and nitrogen-limited growth rates. The presence of bimodal DNA frequency distributions under all conditions confirms that the overlapping rounds of DNA replication that characterize E. coli and other fast-growing prokaryotes are not present in this organism, Although chromosome replication time, C, was constrained to a fairly narrow range of values overall, it nevertheless did vary with growth rate and Limiting factor. Light-limited cells growing at moderate rates had higher C values than did N-limited cells growing at comparable rates (by as much as a factor of 2), As these cells became light saturated, however, C decreased sharply to the level observed under N limitation. The post-replication period, D, decreased monotonically with growth rate under both light and N limitation, approaching a constant value at moderate to high growth rates. Average cell volume at the time of initiation of DNA replication was calculated from the values of C and D, combined with directly measured mean cell volume, and was found to be constant at all growth rates above similar to 0.7 d(-1), This pattern was confirmed by estimates of initiation volume based on flow cytometric light scatter measurements, and suggests that as has been found in other prokaryotic systems, cell mass may play an important role in regulating the timing of chromosome replication in cyanobacteria, Furthermore, because the magnitude of C + D influences average cell mass (given a constant mass at initiation), changes in these parameters (particularly C) may be responsible for the previously reported nonlinear relationship between light-limited growth rate and both RNA cell(-1) and average cell volume.