Studies ofSchizosaccharomyces pombeDNA polymerase α at different stages of the cell cycle

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Abstract
The status of Schizosaccharomyces pombe (fission yeast) DNA polymerase α was investigated at different stages of the cell cycle. S.pombe DNA polymerase a is a phosphoprotein, with serine being the exclusive phosphoamlno acid. By In vivo pulse labeling experiments DNA polymerase a was found to be phosphorylated to a 3-fold higher level In late S phase cells compared with cells in the G2 and M phases, but the steady-state level of phosphorylatlon did not vary significantly during the cell cycle. Tryptlc phosphopeptide mapping demonstrated that the phosphorylation sites of DNA polymerase a from late S phase cells were not the same as that from G2/M phase cells. DNA polymerase a partially purified from G1/S cells had a different mobility in native gels from that from G2/M phase cells. The partially purified polymerase a from G2 phase cells had a higher affinity for singlestranded DNA than that from G2/M phase cells. Despite the apparent differences in cell cycle-dependent phosphorylation, mobility in native gels and affinity for DNA, the in vitro enzymatic activity of the partially purified DNA polymerase a did not appear to vary during the cell cycle. The possible biological significance of these cell cycle-dependent characteristics of DNA polymerase a is discussed.