In Schizosaccharomyces pombe, MBF is a DNA-binding complex suspected to activate the transcription of genes necessary for entry into S phase. The MBF complex contains both p85cdc10 and p72res1/sct1. To obtain a better understanding of how the MBF complex regulates gene expression at the G1/S transition, we have performed a genetic analysis of p72res1. We determined that p72res1 can bind specifically to the cdc22 promoter, when analyzed by gel mobility shift assay, and that the N-terminal 157 amino acids of p72res1 are sufficient for this specific binding. When overexpressed in vivo, a fragment of p72res1 containing this DNA-binding domain could rescue a strain carrying a temperature-sensitive cdc10 allele at the restrictive temperature as well as a strain with a cdc10 null allele. We also determined that the C-terminal region of p72res1 is necessary and sufficient for binding to p85cdc10. Overexpression of the cdc10-binding domain of p72res1 leads to a G1 arrest with a cdc phenotype and a decrease on MBF activity. Overexpression of full-length p72res1 also leads to a growth arrest that can be rescued by overexpression of p85cdc10. These results imply that the MBF activity in vivo is dependent on the interaction of p85cdc10 with p72res1.