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.