Functional Redundancy in the Yeast Cell Cycle: FUS3 and KSS1 Have Both Overlapping and Unique Functions

In the yeast Saccharomyces cerevisiae, cells that pass through a point in the cell cycle called START (Hartwell et al. 1974) are committed to cell division. In the presence of peptide pheromones, haploid cells arrest at START and enter the conjugation pathway (Bucking-Throm et al. 1973). To enter START, cells require a serine/threonine protein kinase, Cdc28 (Reed et al. 1985; Mendenhall et al. 1987; Wittenberg and Reed 1988), which is thought to be activated by G1-specific cyclins, CLN1, CLN2, and CLN3 (Cross et al. 1988; Nash et al. 1988; Richardson et al. 1989; Wittenberg et al. 1990). CLN1, CLN2, and CLN3 are functionally redundant for promoting G1 at START because the presence of any one CLN gene is sufficient for mitotic growth, and the terminal phenotype of a cln1 cln2 cln3 triple mutant is similar to that of cdc28-4 (Reed 1980; Richardson et al. 1989).