DNA synthesis in unfertilized sea urchin eggs can be turned on and turned off by the addition and removal of procaine hydrochloride

Abstract

Unfertilized sea urchin eggs turn on thymidine transport, DNA synthesis and the chromosome cycle in response to procaine hydrochloride. The rates to which these processes activate depend on the extracellular concentration of procaine. Removing procaine turns off DNA synthesis and the chromosome cycle, re-adding procaine turns these processes on once more. Thymidine transport does not turn off after procaine removal. It remains on at the rate it had reached at the time of procaine removal. During a 12-hr period, unfertilized eggs in procaine complete four S-phases, while normal, fertilized embryos complete eight. Tritiated thymidine incorporation into DNA is very low in procaine-treated eggs. This is because in procaine the thymidine transport system is relatively inactive compared to that of fertilized eggs. The data suggest eggs may possess homeostatic mechanisms that actively suppress the DNA synthetic pathway and the chromosome cycle. This system for turning on and off DNA synthesis may prove useful in further analyses of the factors controlling DNA synthesis during early development.