DNA polymerase beta null mouse embryonic fibroblasts harbor a homozygous null mutation in DNA polymerase iota

Abstract

Prior to the explosion of gene targeting technology, considerable biochemical and cell-based experiments suggested a role for DNA polymerase β (pol β) in DNA repair. Since its initial discovery in 1971 [1], the enzyme that was to eventually be designated pol β [2,3] was unique in its enzymatic properties [4–6] as compared to the other newly characterized mammalian DNA polymerases alpha (pol α), gamma (pol γ) and delta (polδ) [1,7]. Of the four eukaryotic DNA polymerases identified by 1977, pol β soon earned the moniker of ‘the’ DNA repair polymerase [8–11]. These early studies defined a role for pol β in repair using isolated nuclei or nuclear extracts, monitoring the incorporation of radioactive nucleosides following ultraviolet (UV) light-induced DNA damage [8–11]. A more definitive role for pol β in excision repair in vitro was demonstrated in 1983, in which complete removal of pyrimidine dimers in DNA was conducted by purified enzyme preparations of pol β, T4 denV (UV endonuclease) and HeLa AP endonuclease II [12]. Although it was subsequently shown that pol α can also carry-out gap-filling DNA synthesis in a base excision repair (BER) reaction [13], the evidence continued to mount in support of pol β acting as ‘the’ DNA repair polymerase in the nucleus, while pol γ participated in replication and repair in mitochondria [1]. Studies continued to identify a role for pol β in the repair of damage induced by many different DNA damaging agents, including bleomycin [14,15], UV-radiation [16], benzo[a]pyrene [17], methylmethane sulfonate [18], ionizing radiation [19], G-T mis-pairs [20] and uracil [21–24]. Several groups reported complete BER in vitro with pol β and additional purified proteins [22,23,25]. Although it was demonstrated in heterologous systems (E. coli and Saccharomyces cerevisiae) that pol β can conduct DNA replication and repair in vivo [26,27], it was not until a mouse gene knockout was made that the specificity of the repair conducted by pol β was defined [28].