INTERACTION BETWEEN THE MEC1-DEPENDENT DNA SYNTHESIS CHECKPOINT AND G1 CYCLIN FUNCTION IN SACCAROMYCES CEREVISIAE

摘要

The completion of DNA synthesis in yeast is monitored by a checkpoint that requires MEC1 and RAD53. Overexpression of the S. cerevisiae G1 cyclin CLN1 results in genomic instability and lethality of mec1-1 checkpoint-deficient cells compared to cln1 cln2 mec1-1 cells. Here we show that overexpression of CLN2 or CLB5, but not CLN3, also killed mec1-1 strains. We identified RNR1, which encodes a subunit of ribonucleotide reductase, as a high-copy suppressor of the lethality of mec1-1 GAL1-CLN1. Northern analysis demonstrated that RNR1 expression is reduced by CLN1 or CLN2 overexpression. Since limiting RNR1 expression might decrease dNTP pools, CLN1 or CLN2 may cause lethality in mec1-1 strains by causing initiation of DNA replication with inadequate dNTP pools. In MEC1 strains, a delay of S phase due to the checkpoint would occur, but in mec1-1 strains lethality would result. Consistent with this model, CLN1 overexpression did result in a prolongation of S phase in a MEC1 background. In a cln1 cln2 background, a prolonged period of expression of genes turned on at the G1-S border has been observed. Thus deletion of CLN1 and CLN2 could function similarly to overexpression of RNR1 in suppressing mec1-1 lethality. While Mec1p has been proposed to act in a linear pathway upstream of Rad53p, rad53 lethality was rescued little by deletion of CLN1 and CLN2, suggesting that Rad53p has Mec1-independent activities.