The metabolic cycle in slowly growing yeast cells. (a) The cycle of stored carbohydrate. In slowly growing cells, glycogen and trehalose build up during G1, then are suddenly liquidated in late G1. Shortly after liquidation, the mRNA levels of the G1 cyclins Cln1 and Cln2 reach a peak, Start is passed, and then budding and DNA synthesis occur. Adapted from the data of Sillje et al. , who studied cells in which the length of G1 was 500 to 600 minutes. (b) The metabolism of spontaneously synchronized cells in limiting glucose. The cyclic changesin the levels of various indicators of metabolism are shown. A small spike of cyclic AMP is seen in mid or late G1. Almost immediately afterwards, glycogen and trehalose are liquidated. Ethanol appears in the medium, presumably the result of fermentation of the freed glucose. The amount of dissolved oxygen in the medium plunges at the same time that stored carbohydrate is disappearing, and the same time as ethanol is appearing. The disappearance of oxygen suggests that glucose from stored carbohydrate isbeing metabolized by respiration as well as by fermentation. The respiratory quotient spikes from just below 1 to about 1.2, signifying a shift from nearly pure respiration (which would give a respiratory quotient of 1.0) tometabolism involving some fermentation. Budding follows shortly afterwards. Part (b) is adapted from a study by Muller et al. , in which cyclic AMP varies from about 6 nmol/g dry weight to about 12 nmol/g dry weight; stored carbohydrate varies from 175 mg/g dry weight to about 80 mg/gdry weight; ethanol varies from 0 to 125 mg/l, dissolved oxygen varies from 80-65% saturation (in Muller et al. ) or 60-20% saturation (in Tu et al., see Figure 1 in ); the respiratory quotient variesfrom 0.85 to 1.2 at the top of the spike; and budding index varies from 5% to 40%.