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 re sistance to ionizing radiation and UV light, as well as regulating cel l cycle progression after irradiation. We have isolated functional rad 9 cognates from two other fission yeasts, Sz. malidevorans and Sz. oct osporus, that can restore radioresistance and the radiation-induced G2 delay response to Sz. pombe rad9::ura4 cells. The Sz. pombe and St. m alidevorans genes are identical at the nucleotide sequence level, whic h reflects their close evolutionary relationship. Each bears three int rons 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 produc t is only 65% identical and 80% similar to the corresponding Sz. octos porus 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 th at the cellular process(es) mediated by rad9, and used by these organi sms to increase survival and transiently delay cycling in G2 after irr adiation, are conserved. The isolation, analyses and comparison of rad 9 genes from different organisms should aid in elucidating the specifi c biological role of the corresponding protein and especially help pin point regions important for function.