SCHIZOSACCHAROMYCES-POMBE CDC20(-POLYMERASE-EPSILON AND IS REQUIRED FOR CHROMOSOMAL REPLICATION BUT NOT FOR THE S-PHASE CHECKPOINT() ENCODES DNA)

In fission yeast both DNA polymerase alpha (pol alpha) and delta (pol delta) are required for DNA chromosomal replication, Here we demonstra te that Schizosaccharomyces pombe cdc20(+) encodes the catalytic subun it of DNA polymerase epsilon (pol epsilon) and that this enzyme is als o required for DNA replication, Following a shift to the restrictive t emperature, cdc20 temperature-sensitive mutant cells block at the onse t of DNA replication, suggesting that cdc20(+) is required early in S phase very near to the initiation step, In the budding yeast Saccharom yces cerevisiae, it has been reported that in addition to its proposed role in chromosomal replication, DNA pol epsilon (encoded by POL2) al so functions directly as an S phase checkpoint sensor [Navas, T. A., Z hou, Z. & Elledge, S. J. (1995) Cell 80, 29-39]. We have investigated whether cdc20(+) is required for the checkpoint control operating in f ission yeast, and our data indicate that po epsilon does not have a ro le as a checkpoint sensor coordinating S phase with mitosis, In contra st, germinating spores disrupted for the gene encoding pol alpha rapid ly enter mitosis in the absence of DNA synthesis, suggesting that in t he absence of pol alpha, normal coordination between S phase and mitos is is lost, We propose that the checkpoint signal operating in S phase depends on assembly of the replication initiation complex, and that t his signal is generated prior to the elongation stage of DNA synthesis .