GENES THAT CAN BYPASS THE CLN REQUIREMENT FOR SACCHAROMYCES-CEREVISIAE CELL-CYCLE START

Cell cycle START in Saccharomyces cerevisiae requires at least one of the three CLN genes (CLN1, CLN2, or CLN3). A total of 12 mutations byp assing this requirement were found to be dominant mutations in a singl e gene that we named BYC1 (for bypass of CLN requirement). We also iso lated a plasmid that had cln bypass activity at a low copy number; the gene responsible was distinct from BYC1 and was identical to the rece ntly described BCK2 gene. Strains carrying bck2::ARG4 disruption allel es were fully viable, but bck2::ARG4 completely suppressed the cbl byp ass activity of BYC1. swi4 and swi6 deletion alleles also efficiently suppressed BYC1 cln bypass activity; Swi4 and Swi6 are components of a transcription factor previously implicated in control of CLN1 and CLN 2 expression. bck2::ARG4 was synthetically lethal with cln3 deletion, suggesting that CLN1 and CLN2 cannot function in the simultaneous abse nce of BCK2 and CLN3; this observation correlates with low expression of CLN1 and CLN2 in bck2 strains deprived of CLN3 function. Thus, fact ors implicated in CLN1 and CLN2 expression and/or function are also re quired for BYC1 function in the absence of all three CLN genes; this m ay suggest the involvement of other targets of Swi4, Swi6, and Bck2 in START.