In budding yeast G1 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 Show more
In budding yeast G1 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 G1-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 G1 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 p40SIC1, 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 G1 demands a rethinking of Start as a single process that causes cells to be committed to the mitotic cell cycle. Show less
Entry into the mitotic cycle (START) requires a protein kinase encoded by the CDC28 gene and one of three redundant G1-specific cyclins encoded by CLN1, -2, and -3. SWI4 and SWI6 are transcription fac Show more
Entry into the mitotic cycle (START) requires a protein kinase encoded by the CDC28 gene and one of three redundant G1-specific cyclins encoded by CLN1, -2, and -3. SWI4 and SWI6 are transcription factors required for the START-dependent activation of the HO endonuclease gene. They also fulfill an overlapping but essential role for cell division since cells deleted for both genes are inviable. We show that the essential role of SWI4 and SWI6 is to ensure the activity of G1-specific cyclin genes. SWI4 and SWI6 appear necessary for the transcription of CLN1 and CLN2, but not for that of CLN3. CLN3 function is, however, also dependent on SWI4 and SWI6. Show less
Yeast cells become committed to the mitotic cell cycle at a stage during G1 called Start. To enter Start, cells must grow to a critical size. They also require the CDC28 protein kinase and at least on Show more
Yeast cells become committed to the mitotic cell cycle at a stage during G1 called Start. To enter Start, cells must grow to a critical size. They also require the CDC28 protein kinase and at least one of three G1-specific cyclins encoded by CLN1, 2, and 3. It is thought that Start is triggered by the accumulation of G1 cyclins that bind to the CDC28 kinase and activate it. So what determines the accumulation of G1 cyclins? For CLN1 and CLN2, transcriptional activation could be involved because their RNAs appear transiently during the cell cycle as cells undergo Start. Here we report that the appearance of CLN1 and CLN2 RNAs depends on an active CDC28 kinase and is stimulated by CLN3 activity. We propose that CDC28 kinase activity due to CLN1 and CLN2 proteins arises through a positive feedback loop which allows CLN proteins to promote their own synthesis. Show less