BIOCHEMICAL AND CYTOLOGICAL EXAMINATION ON INITIATION OF RIBOSOMAL-RNA SYNTHESIS DURING GASTRULATION OF XENOPUS-LAEVIS

Embryos of X. laevis at stage 6 were labeled with 14CO2 for 4 h and then allowed to develop under nonradioactive conditions until they reached stages 9, 10, 11 or 12. RNA was extracted and electrophoresed on a polyacrylamide-agarose gel. From the pattern of newly synthesized RNA, the incorporation into 18S and 28S RNA was measured. The specific radioactivity of nucleoside triphosphates in the acid-soluble fraction was determined. The absolute amounts of the RNA synthesized were calculated. The synthesis of rRNa began, or is at least markedly activated, around stage 10. Cytological examination showed that cells with nucleolated nuclei appeared at stages 9-10 and increased thereafter. The initiation of 18S and 28S RNA synthesis took place around stage 10.