Stimulation by pH of cell cycle initiation in the yeast Candida utilis

Abstract

The effect of shifting pH of the growth medium on cell cycle initiation by the yeast Candida utilis was studied. The yeast was grown by the phased method of cultivation with nitrogen source (ammonium) in growth limiting concentrations and with a phasing period (imposed doubling time) of 6 h. The pH of the culture during the phased growth was maintained between 2.0 and 2.1. The rate of cell cycle initiation as determined by the rate of bud emergence was 24% per hour. If the pH of the culture was shifted to 6.0 at the beginning of the phasing period and maintained at that level for the rest of the phasing period the rate of bud emergence increased to 50% per hour. The increased rate of bud emergence was accompanied by a fast uptake of oxygen and the growth-limiting nutrient and by a reduction in the respiratory quotient. The results suggest that the pH shift accelerated cellular processes necessary for cell cycle initiation.