I t has been shown in a previous paper (Sconzo et al., 1970a) t h a t when fertilized Paracentrotus lividus eggs are exposed to exogenous p3~ for 8 hours, the la t ter becomes incorporated into the nucleotidic pool and tha t the specific ac t iv i ty of the e-phosphates of the nucleotides remains cons tant a t least for fur ther 16 hours. This knowledge enables one to calculate from the a m o u n t of radioac t iv i ty t ransferred wi th in 8 and 24 hours after fert i l izat ion from the pool to accumula t ing RNAs (e.g. ribosomal) the ne t synthesis of RN A in terms of moles. A l imit to this calculat ion was however represented by the possibil i ty t ha t the ac t iv i ty of the e-phosphates in nucleotides monoand di-phosphates was different from tha t of the t r iphosphates and t h a t this difference changed as development proceeded. We have therefore unde r t aken a detai led analysis of the ex ten t of labeling of several nueleotides. This analysis is also of a pract ical value when one considers t h a t the so labeled nucleotidic pool of the sea urch in embryo represents a convenien t source of t r iphosphate nueleotides labeled in the e-phosphate t ha t can be used as substrates for in vitro nucleic acid synthesis.
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