MUTATIONS IN RAD27 DEFINE A POTENTIAL LINK BETWEEN G(1) CYCLINS AND DNA-REPLICATION

The yeast Saccharomyces cerevisiae has three G(1) cyclin (CLN) genes w ith overlapping functions. To analyze the functions of the various CLN genes; we examined mutations that result in lethality in conjunction with loss of cln1 and cln2. We have isolated alleles of RAD27/ERC11/YK L510, the yeast homolog of the gene encoding flap endonuclease 1,FEN-1 . cln1 cln2 rad27/erc11 cells arrest in S phase; this cell cycle arres t is suppressed by the expression of CLN1 or CLN2 but not by that of C LN3 or the hyperactive CLN3-2, rad27/erc11 mutants are also defective in DNA damage repair, as determined by their increased sensitivity to a DNA-damaging agent, increased mitotic recombination rates, and incre ased spontaneous mutation rates, Unlike the block in cell cycle progre ssion, these phenotypes are not suppressed by CLN1 or CLN2. CLN1 and C LN2 may activate an RAD27/ERC11-independent pathway specific for DNA s ynthesis that CLN3 is incapable of activating, Alternatively, CLN1 and CLN2 may be capable of overriding a checkpoint response which otherwi se causes cln1 cln2 rad27/erc11 cells to arrest, These results imply t hat CLN1 and CLN2 have a role in the regulation of DNA replication. Co nsistent with this, GAL-CLN1 expression in checkpoint-deficient, mec1- 1 mutant cells results in both cell death and increased chromosome los s among survivors, suggesting that CLN1 overexpression either activate s defective DNA replication or leads to insensitivity to DNA damage.