RNA synthesis and cytoplasmic polyadenylation in the one-cell mouse embryo.

Abstract

The time of onset of transcriptional activity in the early mammalian embryo is unclear. RNA synthesis has been shown to occur at the two-cell stage in the mouse1–4 and at the two- to four-cell stage in the rabbit5, but the period immediately following fertilization has proved largely inaccessible to biochemical characterization of transcription because of the low permeability of early embryos to exogenous precursors such as 3H-uridine1–7. This difficulty, and the failure to detect RNA polymerase activity in the pronuclei of one-cell mouse embryos8, suggested that the embryonic genome was transcriptionally inactive until some time after the first cleavage. To investigate this issue further, we have now incubated one-cell pronuclear embryos with 3H-adenosine, which is taken up about 1,000 times faster than uridine3. Three labelled RNA species could be identified. The major product is large, heterodisperse, poly(A)− RNA. High-molecular-weight poly(A)+ RNA is also heavily labelled but this is mostly due to cytoplasmic polyadenylation of previously non-polyadenylated, stored RNA. A significant amount of label is incorporated into the -CCA termini of transfer RNA but some new synthesis of tRNA also takes place. No label was found in ribosomal RNA at the one-cell stage but synthesis of mature rRNA species was evident in the two-cell embryo.