Coherence and timing of cell cycle start examined at single-cell resolution.

Cell cycle "Start" in budding yeast involves induction of a large battery of G1/S-regulated genes, coordinated with bud morphogenesis. It is unknown how intra-Start coherence of these events and inter-Start timing regularity are achieved. We developed quantitative time-lapse fluorescence microscopy on a multicell-cycle timescale, for following expression of unstable GFP under control of the G1 cyclin CLN2 promoter. Swi4, a major activator of the G1/S regulon, was required for a robustly coherent Start, as swi4 cells exhibited highly variable loss of cooccurrence of regular levels of CLN2pr-GFP expression with budding. In contrast, other known Start regulators Mbp1 and Cln3 are not needed for coherence but ensure regular timing of Start onset. The interval of nuclear retention of Whi5, a Swi4 repressor, largely accounts for wild-type mother-daughter asymmetry and for variable Start timing in cln3 mbp1 cells. Thus, multiple pathways may independently suppress qualitatively different kinds of noise at Start.