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 mouse^1–4 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-uridine^1–7. 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.