A yeast transcription factor bypassing the requirement for SBF and DSC1/MBF in budding yeast has homology to bacterial signal transduction proteins.

The transcription factors SBF and DSC1/MBF bind SCB and MCB promoter elements, respectively, and are essential for the cell cycle progression of Saccharomyces cerevisiae through the control of G1 cyclin gene expression. We isolated a gene (BRY1; Bacterial Response regulator in Yeast) able to activate either MCB or SCB promoter elements on a reporter plasmid which, when overexpressed, can bypass the normally essential requirement for SBF and DSC1/MBF by the stimulation of CLN1 and CLN2 expression. In the case of CLN2 at least, this expression depends upon the MCB and SCB promoter elements. In wild-type yeast, the disruption of BRY1 has no apparent phenotype, but under conditions where the activities of SBF and DSC1/MBF are reduced, BRY1 becomes essential. Our data imply the existence of a third pathway affecting cyclin expression. BRY1 is the same gene as SKN7 which has significant sequence homology to the receiver domains found in response regulator proteins from the bacterial two-component signal transduction pathways. SKN7 is thought to affect cell wall structure, and when highly overexpressed we find that BRY1/SKN7 is lethal perhaps because of perturbations in cell wall biosynthesis. The lethality is partially rescued by genes from the protein kinase C pathway, but genetic data imply that BRY1/SKN7 and protein kinase C are not in the same pathway. Our results suggest that Bry1/Skn7 can influence the expression of MCB- and SCB-driven gene expression in budding yeast, perhaps including genes involved in cell wall metabolism, via a two-component signal transduction pathway which activates Bry1/Skn7 in response to an unidentified signal.