ROLES AND REGULATION OF CLN-CDC28 KINASES AT THE START OF THE CELL-CYCLE OF SACCHAROMYCES-CEREVISIAE

In budding yeast G(1) cells increase in cell mass until they reach a critical cell size, at which point (called Start) they enter S phase, bud and duplicate their spindle pole bodies. Activation of the Cdc28 protein kinase by G(1)-specific cyclins Cln1, Cln2 or Cln3 is necessary for all three Start events, Transcriptional activation of CLN1 and CLN2 by SBF and MBF transcription factors also requires an active Cln-Cdc28 kinase and it has therefore been proposed that the sudden accumulation of CLN1 and CLN2 transcripts during late G(1) occurs via a positive feedback loop, We report that whereas Cln1 and Cln2 are required for the punctual execution of most, if not all, other Start-related events, they are not required for the punctual activation of SBF- or MBF-driven transcription, Cln3, on the other hand, is essential, By turning off cyclin B proteolysis and turning on proteolysis of the cyclin B-Cdc28 inhibitor p40(SIC1), Cln1 and Cln2 kinases activate cyclin B-Cdc28 kinases and thereby trigger S phase, Thus the accumulation of Cln1 and Cln2 kinases which starts the yeast cell cycle is set in motion by prior activation of SBF- and MBF-mediated transcription by Cln3-Cdc28 kinase. This dissection of regulatory events during late G(1) demands a rethinking of Start as a single process that causes cells to be committed to the mitotic cell cycle.