Abstract
The relation between DNA replication and the action of the mutagen N-methyl-N'-nitro-N-nitroso-guanidine has been studied in Saccharomyces cerevisiae. The frequenceis of reversion to prototrophy of six auxotrophic markers located along one arm of chromosome VII were examined as a function of the vegetative cell cycle. Exponentially growing cells were treated with nitrosoguanidine and then separated by zonal rotor centrifugation into fractions equivalent to stages in the cell cycle. The frequency of reversion for five of the six markers is greatest during the period of DNA replication. Each marker has a single point of maximum reversion, approximately 10-fold greater than the frequency observed at other points in the cell cycle. For any one marker the effect of nitrosoguanidine is restricted to an interval shorter than the period of DNA replication. The two markers most distant from each other, ade5 and leul, both have their highest reversion frequency early during DNA replication. The peak reversion frequency for lys5 is somewhat later, while the peaks for tyr3 and trp5 occur near the end of DNA replication. The results indicate that nitrosoguanidine acts primarily during DNA replication and that different markers appear to be affected at different intervals during the DNA biosynthetic period. If nitrosoguanidine does act at the growing point of DNA replication, these observations indicate that the initiation of DNA replication occurs at specific times during the period of DNA synthesis and at specific initiation sites. Further, there must be more than one point of initiation of DNA replication on one arm of chromosome VII.