THE SCHIZOSACCHAROMYCES-POMBE MBF COMPLEX REQUIRES HETERODIMERIZATIONFOR ENTRY INTO S-PHASE

In Schizosaccharamyces pombe, MBF is a DNA-binding complex suspected t o activate the transcription of genes necessary for entry into S phase . The MBF complex contains both p85(cdc10) and p72(res1/sct1). To obta in a better understanding of how the MBF complex regulates gene expres sion at the G(1)/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 th e N-terminal 157 amino acids of p72(res1) are sufficient for this spec ific binding. When overexpressed in vivo, a fragment of p72(res1) cont aining this DNA-binding domain could rescue a strain carrying a temper ature-sensitive cdc10 allele at the restrictive temperature as well as a strain with a cdc10 null allele. We also determined that the C-term inal region of p72(res1) is necessary and sufficient for binding to p8 5(cdc10). Overexpression of the cdc10-binding domain of p72(res1) lead s to a G(1) arrest with a cde phenotype and a decrease on MBF activity . Overexpression of full-length p72(res1) also leads to a growth arres t that can be rescued by overexpression of p85(cdc10). These results i mply that the MBF activity in vivo is dependent on the interaction of p85(cdc10) With p72(res1).