POSITIVE AND NEGATIVE ROLES FOR CDC10 IN CELL-CYCLE GENE-EXPRESSION

In this paper we describe properties of the cdc10-C4 mutant of the fis sion yeast Schizosaccharomyces pombe. The cdc10(+) gene encodes a comp onent of the DSC1(Sp)/MBF transcription complex, which is required for cell-cycle regulated expression at G(1)-S of several genes via cis-ac ting MCB (Mlul cell cycle box) elements. At permissive temperatures cd c10-C4 causes expression of MCB-reguiated genes through the whole cell cycle, which in asynchronously dividing cells is manifested in overal l higher expression levels. This overexpression phenotype is cold sens itive: in cdc10-C4 cells, MCB genes are expressed at progressively hig her levels at lower temperatures. In heterozygous cdc10-C4/cdc10(+) di ploid strains, MCB-regulated genes are not overexpressed, suggesting t hat loss, rather than alteration, of function of the cdc10-C4 gene pro duct is the reason for unregulated target gene expression. Consistent with this, the cdc10-C4 mutant allele is known to encode a truncated p rotein. We have also overexpressed the region of the cdc10 protein abs ent in cdc10-C4 under the control of an inducible promoter. This induc es a G(1) delay, and additionally causes a reduction of the overexpres sion of MCB genes in cdc10-C4 strains. These results suggest that DSC1 (Sp)/MBF represses, as well as activates, MCB gene expression during t he cell cycle.