RATE OF NUCLEAR RIBONUCLEIC ACID TURNOVER IN SEA URCHIN EMBRYOS

RNA synthesis in the developing sea urchin embryo was characterized by following the incorporation of [3H]guanosine into guanosine triphosphate and the 2′-3′ guanosine monophosphate of hydrolyzed RNA. The amount of guanosine monophosphate in the total RNA which became radioactive was calculated (for a 10-min period), and the turnover values range from 0.7% for a mid-cleavage stage to about 1.3% for the pluteus larva. The rate of RNA synthesis per genome actually decreased as development proceeded, for the rate of cell replication exceeded the increase in RNA synthesis per embryo. The kinetics of labeling of RNA showed that initial incorporation rates were not maintained, and it is probable that a substantial amount of newly made RNA is unstable. Autoradiograms showed that prior to gastrulation much of the newly made RNA remained restricted to the nucleus for at least two hours. It is proposed that labile nuclear RNA is quantitatively an important part of the RNA synthesized during early development. © 1969.