The cell division cycle of the yeast Saccharomyces cerevisiae is triggered at the stage called ‘START’. Many results strongly suggest that adenylate cyclase is an essential element of the control of START. We report here results arguing for a positive control of the cAMP level by the CDC25 gene, another gene of START. Firstly, cdc25 cells can be rescued by extracellular cAMP. Secondly, the cellular cAMP content drops when thermosensitive cdc25 mutant cells are shifted to restrictive temperature. We report the molecular cloning of the CDC25 gene by complementation of cdc25 mutant cells. The identity of the cloned gene was confirmed by site‐specific gene re‐integration experiments and segregation analysis: the isolated fragment is shown to integrate into the cdc25 gene locus. When transferred in cdc25 mutant cells this DNA prevents the drop of the cAMP level at restrictive temperature. This gene is transcribed in a 5200‐nucleotides mRNA. We have determined the nucleotide sequence of a 5548‐bp DNA fragment which shows an uninterrupted open reading frame (ORF) coding for a 1587‐amino acid polypeptide chain. Only the C‐terminal part of the ORF appears to be essential for the complementation of the cdc25‐5 allele, suggesting a multidomain protein.

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