Holoenzyme Forms of Yeast DNA Polymerases δ and ε

Abstract

A detailed understanding of the mechanism of chromosome replication in the eukaryotic cell and of the fidelity of this process requires a comprehensive biochemical and genetic characterization of the DNA polymerases and their accessory factors. Genetic manipulation in the yeast Saccharomyces cerevisiae has shown that at least three nuclear DNA polymerases are essential for cell viability. These are the products of the POL1 (also designated CDC17), POL2, and POL3 (CDC2) genes (Johnson, et al, 1985, Morrison et al., 1990, Sitney et al., 1989, Boulet et al., 1989). Extensive structural and functional similarity between yeast and mammalian cells for many components of the replication apparatus including the replicative DNA polymerases has prompted us to propose a unified nomenclature for the eukaryotic DNA polymerases (Burgers et al., 1990). The catalytic subunit of DNA polymerase a, previously called DNA polymerase I in yeast, is encoded by the yeast POL1 gene (Johnson et al., 1985). Polα is a 4-subunit enzyme and contains an associated DNA primase activity. One still unsolved problem in the biochemistry of Polα is whether this enzyme possesses a proofreading exonuclease activity (reviewed in Burgers, 1989). Whereas the 4-subunit yeast enzyme does not have a detectable 3’-5’-exonuclease activity, the purified monomelic polymerase polypeptide exhibited such an exonuclease activity in one, but not in another study (Kunkel et al., 1989, Brooke et al., 1991). Polδ, previously designated PolIII in yeast, is the product of the yeast POL3 gene (Sitney et al., 1989, Boulet et al., 1989). The 2-subunit enzyme has a 3′-5′-exonuclease activity capable of proofreading in vitro (Bauer, G.A. et al., 1988). The catalytic subunit of Pole, PolII, is the product of the yeast POL2 gene.