Early cell cycle box-mediated transcription of CLN3 and SWI4 contributes to the proper timing of the G(1)-to-S transition in budding yeast

The Cln3-Cdc28 kinase is required to activate the Swi4-Swi6 transcription c omplex which induces CLN1 and CLN2 transcription in late G(1) and drives th e transition to S. Cln3 and Swi4 are both rate limiting for G(1) progressio n, and they are coordinately transcribed to peak at the M/G(1) boundary. Ea rly cell cycle box (ECB) elements, which confer M/G(1)-specific transcripti on, have been found in both promoters, and elimination of all ECB elements from the CLN3 promoter causes both a loss of periodicity and Cln3-deficient phenotypes, which include an extended G(1) interval and increased cell vol ume. Mutants lacking the ECB elements in both the CLN3 and SW14 promoters h ave low and deregulated levels of CLN transcripts, and the G(1)-to-S transi tion for these mutants is delayed and highly variable. These observations s upport the view that the coordinated rise of Cln3 and Swi4 levels mediated by ECB-dependent transcription controls the timing of the G(1)-to-S phase t ransition.