Abstract
Chromosomal DNA replication was examined in temperature-sensitive mutants of Saccharomyces cerevisiae defective in a gene required for the completion of S phase at the nonpermissive temperature, 37 degrees C. Based on incorporation of radioactive precursors and density transfer experiments, strains carrying three different alleles of cdc2 failed to replicate approximately one-third of their nuclear genome at 37 degrees C. Whole-cell autoradiography experiments demonstrated that 93 to 96% of the cells synthesized DNA at 37 degrees C. Therefore, all cells failed to replicate part of their genome. DNA isolated from terminally arrested cells was of normal size as measured on neutral and alkaline sucrose gradients, suggesting that partially replicated DNA molecules do not accumulate and that DNA strands are ligated properly in cdc2 mutants. In addition, electron microscopic examination of the equivalent of more than one genome's DNA from arrested cells failed to reveal any partially replicated molecules. The sequences which failed to replicate at 37 degrees C were not highly specific; eight different cloned sequences replicated to the same extent as total DNA. The 2-microns plasmid DNA and rDNA replicated significantly less well than total DNA, but approximately one-half of these sequences replicated at 37 degrees C. These observations suggest that cdc2 mutants are defective in an aspect of initiation of DNA replication common to all chromosomes such that a random fraction of the chromosomes fail to initiate replication at 37 degrees C, but that once initiated, replication proceeds normally.