Spo12 is a limiting factor that interacts with the cell cycle protein kinases Dbf2 and Dbf20, which are involved in mitotic chromatid disjunction.

The DBF2 and DBF20 genes of the budding yeast Saccharomyces cerevisiae encode a pair of structurally similar protein kinases. Although yeast with either gene deleted is viable, deletion of both genes is lethal. Thus, the Dbf2 and Dbf20 proteins are functional alternatives for an essential activity. In contrast to deletions, four different mutant alleles of DBF2 are lethal. Thus, the presence of a nonfunctional Dbf2 protein, rather than the lack of function per se, is inhibitory. Here we present genetic evidence that nonfunctional mutant Dbf2 protein blocks the function of Dbf20 protein by sequestering a common interacting protein encoded by SPO12. Even a single extra copy of SPO12 is sufficient to suppress the dbf2 defect. Since SPO12 appears to encode a limiting factor, it may be a rate limiting cofactor that is involved in the regulation of the Dbf2 and Dbf20 protein kinases. A corollary to the finding that one extra copy of SPO12 can suppress dbf2, is that the acquisition of an extra chromosome VIII, which carries the SPO12 locus, will also suppress dbf2. Indeed, physical analysis of chromosome copy number in dbf2 revertants able to grow at 37 degrees showed that the frequency of chromosome VIII acquisition increased when cells were incubated at the restrictive temperature, and reached a frequency of more than 100-fold the amount in wild-type yeast. This suggested that the dbf2 mutation was not only suppressed by an extra copy of chromosome VIII but also that the dbf2 mutation actually caused aberrant chromosomal segregation. Conventional assays for chromosome loss confirmed this proposal.