Cell cycle regulation and the timing of chromosome replication in a marineSynechococcus (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. Th e presence of bimodal DNA frequency distributions under all conditions conf irms that the overlapping rounds of DNA replication that characterize E. co li and other fast-growing prokaryotes are not present in this organism, Alt hough chromosome replication time, C, was constrained to a fairly narrow ra nge of values overall, it nevertheless did vary with growth rate and Limiti ng factor. Light-limited cells growing at moderate rates had higher C value s than did N-limited cells growing at comparable rates (by as much as a fac tor of 2), As these cells became light saturated, however, C decreased shar ply to the level observed under N limitation. The post-replication period, D, decreased monotonically with growth rate under both light and N limitati on, approaching a constant value at moderate to high growth rates. Average cell volume at the time of initiation of DNA replication was calculated fro m 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 flo w cytometric light scatter measurements, and suggests that as has been foun d in other prokaryotic systems, cell mass may play an important role in reg ulating the timing of chromosome replication in cyanobacteria, Furthermore, because the magnitude of C + D influences average cell mass (given a const ant mass at initiation), changes in these parameters (particularly C) may b e responsible for the previously reported nonlinear relationship between li ght-limited growth rate and both RNA cell(-1) and average cell volume.