A second growth state for Schizosaccharomyces pombe.

The kinetics of volume increase in individual cells of Schizosaccharomyces pombe were determined by phase microscopy at osmolalities lower than those reported in the literature. At the highest osmolality, 550 mmol/kg, all cells followed a biphasic pattern of growth, in which cell volumes increased to their maximum values approximately four-fifths of the way through the growth cycle. At lower osmolalities (400-420 mmol/kg), many or most of the cells followed a different growth pattern, with a linear increase in cell volume throughout the cycle. The following evidence indicates that a different regulatory mechanism is responsible for the linear growth pattern: (1) Regulation of cell length and diameter differed for the two cases. During biphasic growth, cell length also increased biphasically and cell diameters remained essentially constant during the cycle, whereas during linear growth, both cell length and diameter increased linearly until formation of the cell plate very late in cycle. (2) The two different growth states were observed for cells growing on two very different kinds of medium. (3) Frequency distributions of the two growth patterns showed that there were two distinct groups of growing cells, with and without a cell volume plateau; these results rule out a single growth state in which plateaus are graded from large to infinitesimally small. (4) Linear regressions fitted to the data for linear growth did not differ significantly from the theoretical model for linear growth without a terminal plateau. These results reveal the operation of a second regulatory system for cell growth in S. pombe at osmolalities closer to those in liquid medium. The occurrence of transitions between the two growth states in successive generations and the agreement between several growth parameters for the two modes suggest that the growth states are closely related.