The Schizosaccharomyces pombe MBF complex requires heterodimerization for entry into S phase

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 p85(cdc10) and p72(res1/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 p72(res1). We determined that p72(res1) can bind specifically to the cdc22 promoter, when analyzed by gel mobility shift assay, and that the N-terminal 157 amino acids of p72(res1) are sufficient for this specific binding. When overexpressed in vivo, a fragment of p72(res1) 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 p72(res1) is necessary and sufficient for binding to p85(cdc10). Overexpression of the cdc10-binding domain of p72(res1) leads to a G₁ arrest with a cdc phenotype and a decrease on MBF activity. Overexpression of full-length p72(res1) also leads to a growth arrest that can be rescued by overexpression of p85(cdc10). These results imply that the MBF activity in vivo is dependent on the interaction of p85(cdc10) with p72(res1).