The yeast analog of mammalian cyclin/proliferating-cell nuclear antigen interacts with mammalian DNA polymerase δ

DNA polymerase III from Saccharomyces cerevisiae is analogous to the mammalian DNA polymerase δ by several criteria, including an increased synthetic activity on poly(dA)·oligo(dT) (40:1 nucleotide ratio) in the presence of calf thymus proliferating-cell nuclear antigen (PCNA), or cyclin. This stimulation assay has been used to purify the yeast analog of PCNA/cyclin (yPCNA) to homogeneity·yPCNA is a trimer or tetramer (M(r) ~ 82,000) of identical subunits with a denatured M(r) of 26,000. On a molar basis yPCNA and calf thymus PCNA/cyclin are equally active in stimulating DNA synthesis by DNA polymerase III. About 10 times more yPCNA than calf thymus PCNA/cyclin is needed, however, to stimulate calf thymus DNA polymerase δ, and the degree of stimulation obtained at saturating levels of yPCNA is a factor of 2-3 less than with calf thymus PCNA/cyclin. Both stimulatory proteins exert their effect in an identical fashion, i.e., by increasing the processivity of the DNA polymerase. Yeast DNA polymerases I and II and calf thymus DNA polymerase α are not stimulated by yPCNA. Treatment of logarithmic-phase cells with hydroxyurea blocks them in the S phase and produces a 4- to 5-fold increase in yPCNA.