Mutations in SID2, a novel gene in Saccharomyces cerevisiae, cause synthetic lethality with sic1 deletion and may cause a defect during S phase

SIC1 encodes a nonessential B-type cyclin/CDK inhibitor that functions at t he G1/S transition and the exit from mitosis. To understand more completely the regulation of these transitions, Mutations causing synthetic lethality with sic1 Delta were isolated. In this screen, we identified a novel gene, SID2, which encodes an essential protein that appear., to be required for DNA replication or repair. sid2-1 sic1 Delta strains and sid2-21 temperatur e-sensitive strains arrest preanaphase as large-budded cells with a single nucleus, a short spindle, and an similar to 2C DNA content. RAD9, which is necessary for the DNA damage checkpoint, is required for the preanaphase ar rest of sid2-1 sic1 Delta cells. Analysis of chromosomes in mutant sid2-21 cells by field inversion gel clectropiioresis suggests the presence of repl ication forks and bubbles at the arrest. Deleting the two S phase cyclins, CLB5 and CLB6, substantially suppresses the sid2-1 sir1 Delta inviability, while stabilizing Clb5 protein exacerbates the defects of sid2-1 sic1 Delta , cells. In synchronized sid2-1 mutant strains, the onset of replication ap pears normal, but completion of DNA synthesis is delayed. sid2-1 mutants ar e sensitive to hydroxytirea indicating that sid2-1 cells may suffer DNA dam age that, when combined with additional insult, leads to a decrease in viab ility. Consistent with this hypothesis, sid2-1 rad9 cells are dead or very slow growing even when SIC1 is expressed.