Interaction between the MEC1-dependent DNA synthesis checkpoint and G1 cyclin function in Saccharomyces cerevisiae

The completion of DNA synthesis in yeast is monitored by a checkpoint that requires MEC1 and RAD53. Here we show that deletion of the Saccharomyces ce revisiae G1 cyclins CLN1 and CLN2 suppressed the essential requirement for MEC1 function. Wild-type levels of CLN1 and CLN2, or overexpression of CLN1 , CLN2, or CLB5, but not CLN3, killed mec1 strains. We identified RNR1, whi ch encodes a subunit of ribonucleotide reductase, as a high-copy suppressor of the lethality of mec1 GAL1-CLN1. Northern analysis demonstrated that RN R1 expression is reduced by CLN1 or CLN2 overexpression. Because limiting R NR1 expression would be expected to decrease dNTP pools, CLN1 and CLN2 may cause lethality in mec1 strains by causing initiation of DNA replication wi th inadequate dNTPs. In contrast to mec1 mutants, MEC1 strains with low dNT Ps would be able to delay S phase and thereby remain viable. We propose tha t the essential function for MEC1 may be the same as its checkpoint functio n during hydroxyurea treatment, namely, to slow S phase when nucleotides ar e limiting. In a cln1 cln2 background, a prolonged period of expression of genes turned on at the G1-S border, such as RNR1, has been observed. Thus d eletion of CLN1 and CLN2 could function similarly to overexpression of RNR1 in suppressing mec1 lethality.