A yeast transcription factor activating G1 cyclin expression has similarity to bacterial signal transduction proteins.

Introduction Yeast cells become committed to enter and complete the mitotic cell cycle at a point known as Start in late G1. At Start a transcriptional programme is initiated, involving at least two transcription factors, SBF and DSCUMBF (for a review see [1,2]). These factors bind related promoter sequences known as SCB and MCB elements respectively. SBF controls principally the G1 cyclins CL,Yl, CLALV2, PCLl and PCL2 [3-51 whereas DSCl/MBF controls a much larger group of genes involved in the execution of S phase [1,6]. CL,Nl and CLN2 have both SCB and MCB elements in their promoters and, at least under some circumstances (see below), either SBF or DSCl/MBF can control their expression. This functional overlap is reflected in the genetics of the proteins constituting the transcription factors. The SBF component recognizing and binding to SCB elements is encoded by the S W 4 gene [7], whereas the MCB binding protein of DSCl/MBF is encoded by MBPl [8]. Swi4 and Mbpl are related proteins showing substantial homology, including their DNA-binding domains. Either gene can be deleted without serious phenotypic consequences, but deletion of both genes is lethal [8]. This lethality can be relieved by the ectopic expression of either CLNl or CI,N2, indicating that the essential function of Swi4 and Mbpl is the expression of G1 cyclins and the consequent activation of the cyclindependent protein kinase Cdc28 [Z]. The transcription factors SBF and DSC 1/ MBF are further related in a shared component encoded by the S W 6 gene [9-111. Swi6 has no specific affinity for SCB or MCB elements [8,12] and may serve to activate SBF and DSCl as cells traverse late G1, ensuring that target genes are