Temporal order in yeast chromosome replication.

Previous work with bacteria has shown that a gene is maximally sensitive to mutagenesis by N-methyl-N'-nitro-N-nitrosoguanidine (NG) at the time it is being replicated. NG was used to test for temporal order in the replication of the genome of the unicellular eucaryote, Saccaromyces cerevisiae. Yeast cells growing exponentially were more sensitive to mutagenesis by NG than cells in which DNA synthesis had been inhibited. Further, in a synchronized population of cells, individual genetic markers exhibited maximum sensitivity to muta-genesis at distinct limited intervals within the DNA synthesis period. The peaks of sensitivity are interpreted as reflecting the times of replication of different genes. Since markers for five genes on four different chromosomes showed discrete periods of maximum sensitivity, it is likely that temporal ordering of replication exists for most genes in the yeast genome. These results imply that sites for initiation of DNA replication occur at fairly specific regions along yeast chromosomal DNA moleucles, and are activated at predetermined times in the DNA synthesis period.