ISOLATION OF A CDC25 FAMILY GENE, MSI2/LTE1, AS A MULTICOPY SUPPRESSOR OF IRA1

We have identified MS12 as a gene of Saccharomyces cerevisiae which, when on a multicopy vector, suppresses the heat shock sensitivity caused by the loss of the IRA1 product, a negative regulator of the RAS protein. The multicopy MS12 also suppresses the heat shock sensitivity of cells with the RAS2(vol19) mutation but not those with the bcyl mutation, suggesting that the MS12 protein may interfere with the activity of the RAS protein. The sequence analysis of MS12 reveals that it is identical to LTE1 belonging to the CDC25 family: CDC25, SCD25 and BUDS, each of which encodes a guanine nucleotide exchange factor for the ras superfamily gene products. Deletion of the entire MS12 coding region reveals that MS12 is not essential but the disruptant shows a cold-sensitive phenotype. Under the non-permissive conditions, more than 70% of the msi2 disruptants arrested at telophase as large budded cells with two nuclei divided completely and elongated spindles, indicating that the msi2 deletion is a cell division cycle mutation. These results suggest that MS12 is involved in the termination of M phase and that this process is regulated by a ras superfamily gene product.