ENTRY of yeast cells into the mitotic cell cycle (Start) involves a form of the CDC28 kinase that associates with Gl-specific cyclins encoded by CLN1 and CLN2 (ref. 1). The onset of Start may be triggered by the activation of CLN1 and CLN2 transcription in late Gl (ref. 2). SWI4 and SWI6 are components of a factor (SBF) that binds the CACGAAAA (SCB) promoter elements3-5 responsible for activation in late Gl of the HO endonuclease, CLN1 and CLN2 genes6,7. A related factor (MBF) containing SWI6 and a 120K protein8 binds to the ACGCGTNA (MCB) promoter elements responsible for late Gl-specific transcription of DNA replication genes9-12. Nothing is known about how these heteromeric proteins bind DNA. We show here that SWI4 contains a novel DNA-binding domain at its N terminus that alone binds specifically to SCBs and a C-terminal domain that binds to SWI6. SWI4's DNA-binding domain is similar to an N-terminal domain of the cdclO protein that is a component of an MBF-like factor from Schizosaccharomyces pombe13 and is required for Start14,15. An involvement of this kind of DNA-binding domain in transcriptional controls at Start may therefore be a conserved feature of eukaryotic cells.
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