A control acting over the initiation of DNA replication in the yeast Schizosaccharomyces pombe

The control of cell division in the yeast Schizosaccharomyces pombe appears to be quite different to that of any other eukaryotic organisms for it is usually exerted not at the initiation of S-phase but at that of mitosis. However, it has been suggested that a control over the initiation of S-phase does also exist but that its action is redundant whilst the mitotic control is operating. This study has chosen conditions in which the latter appears to be largely absent in order to study the cryptic S-phase control. The timing of S-phase has been studied in cells grown at varying rates under nitrogen limitation in a chemostat. It is found that under these conditions the control of cell division resembles that of other eukaryotes. As the dilution rate of the chemostat is reduced, all increase in the generation time can be accounted for by a lengthened G1 period. In contrast, the length of S + G2 remains invariant. Thus, there must indeed be a control acting in G1 in S. pombe. An analysis of the size of cells at different growth rates shows that the initiation of S-phase is correlated with a particular cell size.