[Effect of methylnitronitrosoguanidine on DNA synthesis in animal cells: decrease in the rate of replication fork movement].

The effect of methylnitronitrosoguanidine (MNNG) on the rate of DNA replication fork (RF) progress has been studied by DNA fiber autoradiography in asynchronous Microtus agrestis and Chinese hamster cells. The rate of RF progress has been shown to be decreased by 14% and 36% at MNNG concentrations of 50 and 100 microM in M. agrestis cells; the rate of DNA synthesis being reduced by 50 and 75% respectively. In Chinese hamster cells the MNNG concentration of 5 microM does not affect the rate of RF and that of 10 microM decreases the latter by 11%, the respective fall in the synthesis of DNA rate being 13 and 57%. It has been concluded that the decreased RF rate contributes only partially to the overall DNA synthesis inhibition following the MNNG administration. Inhibition of DNA synthesis at the MNNG concentrations reducing the DNA synthesis by less then 40-50% is mainly caused by the inhibition of the initiation points and, possibly, by the stopping of operating RF. Further DNA synthesis inhibition (at the MNNG concentrations leading to DNA synthesis decrease exceeding 50%) is mostly due to the reduced RF progress rate.