BIOCHEMICAL AND CYTOLOGICAL EXAMINATION ON THE INITIATION OF RIBOSOMAL RNA SYNTHESIS DURING GASTRULATION OF XENOPUS...

Embryos of Xenopus laevis at stage 6 were labeled with 14 CO2 for 4 hr and then allowed to develop under nonradioactive conditions until they reached stage 9, 10, 11 or 12. RNA was extracted and electrophoresed on a polyacrylamide-agarose gel. From the pattern of newly synthesized RNAs, the incorporation into 18S and 28S ribosomal RNAs was measured. At the same time, the specific radioactivity of nucleoside triphosphates in the acid-soluble fraction was determined. On the basis of the results obtained, the absolute amounts of the RNAs synthesized were calculated. The results show that the synthesis of the ribosomal RNAs begins, or is at least markedly activated, around stage 10. Moreover, cytological examination has shown that cells with nucleolated nuclei appeared between stages 9 and 10 and increased thereafter. Thus, from the results of these studies along two different lines, it can safely be concluded that the initiation of 18S and 28S RNA synthesis takes place around stage 10.