Isolation and identification of the third subunit of mammalian DNA polymerase delta by PCNA-affinity chromatography of mouse FM3A cell extracts

Using proliferating cell nuclear antigen affinity chromatography and glycer ol gradient centrifugation of partially purified fractions from mouse FM3A cells we have been able to isolate novel complexes of DNA polymerase delta and DNA ligase 1 containing clearly defined subunit compositions. In additi on to the well known catalytic subunit of 125 kDa and accessory subunit of 48 kDa, the DNA polymerase delta complex contained three supplementary comp onents, one of which reacted with antibodies directed against the p40 and p 37 subunits of RF-C. Of the two remaining components, one termed p66 turned out to be coded by a gene whose putative C-terminal domain displayed signi ficant homology with that of the Cdc27 subunit of Schizosaccharomyces pombe polymerase delta. On the basis of these and other observations, we propose p66 to be the missing third subunit of mammalian DNA polymerase delta. The DNA ligase 1 complex was made up of three novel components in addition to the 125 kDa catalytic subunit, two of which, p48 and p66, were common to DN A polymerase delta. We discuss the implications of our findings within the current framework of our understanding of DNA replication.