Nucleic acid synthesis in preimplantation rabbit embryos. II. Delayed synthesis of ribosomal RNA

The qualitative aspects of ribonucleic acid (RNA) synthesis in preimplantation rabbit embryos have been investigated by the electrophoretic separation of tritium‐labeled embryonic RNA on polyacrylamide gels. Labeling of an RNA species having migratory properties identical with those of transfer RNA can be detected from the 2‐cell stage onward. Labeling of RNA species having the migratory properties of 28S and 18S ribosomal RNA is first detectable at 72 hours post coitum , at the time of the morula‐to‐blastocyst transformation. Labeling of heterogeneous RNA migrating between the 4S and 18S markers is detectable at all stages. The sensitivity of this labeling to inhibition by Actinomycin D is consistent with DNA‐dependent synthesis. The findings demonstrate that cleavage of the rabbit ovum can be accomplished in the absence of significant ribosomal RNA synthesis, and provide evidence that at least one gene product sufficient to sustain early protein synthesis is supplied by the maternal genome prior to fertilization. The distinct temporal separation in the initiation of transfer and ribosomal RNA synthesis is consistent with their independent regulation in these embryos.