The CDC25 "Start" gene of Saccharomyces cerevisiae: sequencing of the active C-terminal fragment and regional homologies with rhodopsin and cytochrome P450.

The CDC25 Start gene whose product appears to be required for traversing the Go phase of the cell cycle in Saccharomyces cerevisiae, has been previously cloned (J. Daniel and G. Simchen (1986), Curr Genet 10:643-646). By nucleotide sequencing of an active subclone, we found that only a region of the gene that codes for the C-terminal portion of the CDC25 protein was required for full suppression of the cdc25 mutation. The codon usage in this region indicates a poor translation of the transcript compared to genes encoding abundant proteins. The derived CDC25 protein fragment contains two regions of homology, one with the rhodopsin family, the other with the cytochrome P450 family. Strikingly, these two regions of homology are adjacent on the CDC25 protein. In view of the likely involvement of the CDC25 protein in the regulation of adenylate cyclase activity, a working hypothesis is proposed that accounts for the observed homologies.