CONDITIONAL LETHALITY OF NULL MUTATIONS IN RTH1 THAT ENCODES THE YEAST COUNTERPART OF A MAMMALIAN 5'-EXONUCLEASE 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 rth1 Delta mutation on various cellular processes. rth1 Delta mutants grow poorly at 30 degrees C, and a cessation in growth occurs upon transfer of the mutant to 37 degrees C. At the restrictive temperature, the rth1 Delta 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 rth1 Delta mutant even at the permissive temperature, The rth1 Delta mutation does not affect UV or gamma-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.