SWI6 protein is required for transcription of the periodically expressed DNA synthesis genes in budding yeast.

In budding yeast many genes are expressed under cell-cycle control in late G1. These include a large group of DNA synthesis genes, the HO gene involved in mating-type switching, CTS1 (chitinase) and also CLN1 and CLN2 (ref. 4) encoding G1 cyclins. Two factors, encoded by the SWI4 and SWI6 genes, are required for HO (ref. 5), CLN (refs 6, 7) and CTS1 (ref. 3) gene expression and, at least in the HO promoter, bind to CACGA4 upstream sequences (CCBs). This motif is not found upstream of the DNA synthesis genes, which instead have a hexamer element, ACGCGT1 (MCB), an MluI restriction site, that is recognized by a cell-cycle regulated transcription complex DSC1 (ref. 1). This MluI-activation system consisting of the MCBs and DSC1 is conserved in fission yeast where a DSC1-like complex controls the cdc22+ ribonucleotide reductase gene. The Schizosaccharomyces pombe cdc10+ gene encodes a component of DSC1 (ref. 10) and, significantly, this has homology with both the Swi4 and Swi6 proteins. Here we show that Swi6 is an essential component of DSC1 and that deletion of SWI6 impairs the cell-cycle regulation of the DNA synthesis genes, as well as CLN1 and CLN2. Thus Swi6 is the common factor in regulation of all the above genes and may therefore be responsible for the timing of their expression in late G1.