Conditional lethality of null mutations in RTH1 that encodes the yeast counterpart of a mammalian 5′- to 3′-exonuclease required for lagging strand DNA synthesis in reconstituted systems

A 5′- to 3′-exonuclease of about 45 kDa has been purified from various mammalian sources and shown to be required for the completion of lagging strand synthesis in reconstituted DNA replication systems. RTH1 encodes the yeast Saccharomyces cerevisiae counterpart of the mammalian enzyme. To determine the in vivo biological role of RTH1-encoded 5′- to 3′-exonuclease, we have examined the effects of an rth1Δ mutation on various cellular processes. rth1Δ mutants grow poorly at 30°C, and a cessation in growth occurs upon transfer of the mutant to 37°C. At the restrictive temperature, the rth1Δ mutant exhibits a terminal cell cycle morphology similar to that of mutants defective in DNA replication, and levels of spontaneous mitotic recombination are elevated in the rth1Δ mutant even at the permissive temperature. The rth1Δ mutation does not affect UV or γ-ray sensitivity but enhances sensitivity to the alkylating agent methyl methanesulfonate. The role of RTH1 in DNA replication and in repair of alkylation damage is discussed.