Commitment to division in Paramecium: effect of nutrient level on the macronuclear DNA increment.

In Paramecium, a fixed macronuclear DNA increment is associated with commitment to cell division. This study shows that this threshold DNA increment is about 70% of the final DNA increment in well-fed cells. The DNA increment is reduced when growth rate is decreased and decreases in parallel with growth rate to a minimum of 30% of the normal DNA increment. This minimum value is obtained when the growth rate is 20% of its normal level or lower. Further reduction in the growth rate produces no further reduction in the DNA increment. Following abrupt nutrient-level shifts, both the threshold DNA increment and the final DNA increment change progressively as the time of the shift is moved to later stages of the cell cycle. The threshold DNA increment is reset following nutrient-level shifts up to the point of commitment to division. These observations are consistent with the notion that the magnitude of the threshold DNA increment is strongly correlated with the rate of growth and is rapidly reset by factors which alter the growth rate. The implications of these observations for growth-driven regulation of the cell cycle are discussed.