In the fission yeast Schizosaccharomyces pombe, the execution of Start
requires the activity of the Cdc2 protein kinase and the Cdc10/Sct1 t
ranscription complex. The loss of any of these genes leads to G(1) arr
est and activation of the mating pathway under appropriate conditions.
We have undertaken a genetic and biochemical analysis of these genes
and their protein products to elucidate the molecular mechanism that g
overns the regulation of Start. We demonstrate that serine-196 of Cdc1
0 is phosphorylated in vivo and provide evidence that suggests that ph
osphorylation of this residue is required for Cdc10 function. Substitu
tion of serine-196 of Cdc10 with alanine (Cdc10 S196A) leads to inacti
vation of Cdc10. We show that Cdc10 S196A is incapable of associating
with Sct1 to form a heteromeric complex, whereas substitution of this
serine with aspartic acid (S196D) restores DNA-binding activity by all
owing Cdc10 to associate with Sct1. Furthermore, we demonstrate that C
dc2 activity is required for the formation of the heteromeric Sct1/Cdc
10 transcription complex and that the Cdc10 S196D mutation alleviates
this requirement. We thus provide biochemical evidence to demonstrate
one mechanism by which the Cdc2 protein kinase may regulate Start in t
he fission yeast cell cycle.