Enzymes of DNA biosynthesis in developing sea urchins: Changes in ribonucleotide reductase, thymidine, and thymidylate kinase activities

Abstract

The pattern of ribonucleotide reductase, thymidine kinase, and thymidylate kinase activities during development of Paracentrotus lividus eggs and the effect of actinomycin on these enzymatic activities have been studied. Ribonucleotide reductase activity is detectable, though at a low level, in the unfertilized egg; the activity increases sharply soon after fertilization and reaches a peak at the morula stage. Thereafter it decreases and remains at a lower level than that of the unfertilized egg. Actinomycin, at a concentration sufficient to inhibit messenger RNA (mRNA) synthesis does not affect the level of enzymatic activity, indicating that preexisting maternal mRNA is used for the synthesis of this enzyme. Thymidine kinase is present at a low level in the egg; it increases sharply after the hatching blastula until the pluteus stage. Actinomycin does not affect the enzyme activity from fertilization until blastula but prevents the increase in enzyme activity that is observed between blastula and pluteus. Thymidylate kinase activity shows an increase after fertilization, followed by fluctuations throughout development with a considerable decrease at the blastula stage and at the end of gastrulation. Actinomycin has no effect on the activity of thymidylate kinase regardless of when the drug is added to the embryo suspension. Possible regulatory mechanisms of DNA synthesis in sea urchin embryos are discussed: The presence in the unfertilized egg of the most important enzymes controlling the cellular flow of DNA precursors and the availability of dTTP suggest that the block in DNA synthesis observed in the unfertilized egg is due to some particular mechanism that is switched on at fertilization.