Schizosaccharomyces pombe cdc20⁺ encodes DNA polymerase ε and is required for chromosomal replication but not for the S phase checkpoint

In fission yeast both DNA polymerase alpha (pol α) and delta (pol δ) are required for DNA chromosomal replication. Here we demonstrate that Schizosaccharomyces pombe cdc20⁺ encodes the catalytic subunit of DNA polymerase epsilon (pol ε) and that this enzyme is also required for DNA replication. Following a shift to the restrictive temperature, cdc20 temperature-sensitive mutant cells block at the onset of DNA replication, suggesting that cdc20⁺ is required early in S phase very near to the initiation step. In the budding yeast Saccharomyces cerevisiae, it has been reported that in addition to its proposed role in chromosomal replication, DNA pol ε (encoded by POL2) also functions directly as an S phase checkpoint sensor [Navas, T. A., Zhou, Z. and Elledge, S. J. (1995) Cell 80, 29-39]. We have investigated whether cdc20⁺ is required for the checkpoint control operating in fission yeast, and our data indicate that pole does not have a role as a checkpoint sensor coordinating S phase with mitosis. In contrast, germinating spores disrupted for the gene encoding pol α rapidly enter mitosis in the absence of DNA synthesis, suggesting that in the absence of pol α, normal coordination between S phase and mitosis is lost. We propose that the checkpoint signal operating in S phase depends on assembly of the replication initiation complex, and that this signal is generated prior to the elongation stage of DNA synthesis.