Transcription factors important for starting the cell cycle in yeast

Publisher Website
Google Scholar
Abstract
Unlike early embryonic cleavage divisions in certain animals, cell-cycle progression in yeast and probably also in all metazoan somatic cells requires the periodic transcriptional activation of certain key genes. Thus far, the only clear examples are genes that encode a class of unstable ‘cyclin’ proteins, which bind and activate the cdc2/Cdc28 protein kinase: the G1-specific cyclins encoded byCLN1andCLN2, a B-type cyclin implicated in DNA replication encoded byCLB5; and four B-type cyclins involved in mitosis encoded byCLB1, 2, 3, 4. CLN1, CLN2, andCLB5are transcribed in late G1, as cells undergo Start. A transcription factor composed of Swi4 and Swi6 proteins (called SBF) activatesCLN1andCLN2transcription via a positive feedback loop in which Cln proteins activate their own transcription. A different but related transcription factor called MBF seems responsible for the late G1-specific transcription of most DNA replication genes includingCLB5. We have purified MBF and shown that it contains Swi6 and a 110-120 kDa protein distinct from Swi4 (pl20) that contacts DNA. Thus, we propose that SBF and MBF share a common regulatory subunit (Swi6) but recognize their promoter elements via distinct DNA binding subunits.