SCHIZOSACCHAROMYCES MALIDEVORANS AND SZ-OCTOSPORUS HOMOLOGS OF SZ-POMBE RAD9, A GENE THAT MEDIATES RADIORESISTANCE AND CELL-CYCLE PROGRESSION

The rad9 gene of Schizosaccharomyces pombe is involved in promoting resistance to ionizing radiation and UV light, as well as regulating cell cycle progression after irradiation. We have isolated functional rad9 cognates from two other fission yeasts, Sz. malidevorans and Sz. octosporus, that can restore radioresistance and the radiation-induced G2 delay response to Sz. pombe rad9::ura4 cells. The Sz. pombe and St. malidevorans genes are identical at the nucleotide sequence level, which reflects their close evolutionary relationship. Each bears three introns and codes for a 47 464-Da protein that contains 426 amino acids (aa). In contrast, Sz. octosporus rad9 contains five introns and codes for a 48 210-Da protein that is 432-aa long. The Sz. pombe rad9 product is only 65% identical and 80% similar to the corresponding Sz. octosporus gene product. All of the strains synthesize a rad9 RNA of approx. 1.6 kb. The presence of a rad9-like gene in these yeasts suggests that the cellular process(es) mediated by rad9, and used by these organisms to increase survival and transiently delay cycling in G2 after irradiation, are conserved. The isolation, analyses and comparison of rad9 genes from different organisms should aid in elucidating the specific biological role of the corresponding protein and especially help pinpoint regions important for function.