Control of cell division in Paramecium tetraurelia. Effects of abrupt changes in nutrient level on accumulation of macronuclear DNA and cell mass.

In the cell cycle of Paramecium there are three points of interaction between cell growth-related processes and the processes of macronuclear DNA replication and cell division: initiation of DNA synthesis, regulation of the rates of growth and DNA accumulation, and initiation of cell division. This study examines the regulation of the latter two processes by analysis of the response of each to abrupt changes in nutrient level brought about either by transferring dividing cells from a steady-state chemostat culture to medium with unlimited food, or by transferring well-fed dividing cells to exhausted medium. The rates of DNA accumulation and cell growth respond quickly to changes in nutrient level. The amounts of these cell components accumulated during the cell cycle following a shift in nutrient level are typical of those occurring during equilibrium growth under post-shift conditions. Commitment to division occurs at a fixed interval prior to fission that is similar in well-fed and nutrient-limited cells. Initiation of cell division in Paramecium is associated with accumulation of a threshold DNA increment, whose level is largely independent of nutritive conditions. The amount of DNA accumulated during the cell cycle varies with nutritional conditions because the rates of growth and DNA accumulation are affected by nutrient level; slowly growing cells accumulated relatively little DNA during the fixed interval between commitment to cell division and fission.