Post-transcriptional control in the early mouse embryo.

The earliest stages of mouse embryogenesis, from fertilisation to the two-cell stage, are characterised by an extremely low level of RNA synthesis. Indeed, during this period, RNA polymerase II activity and incorporation of labelled precurosrs into heterogeneous RNA are not detectable, and there is no increase in the poly(A) content of the embryo, but rather a slight decrease. The rate of protein synthesis remains low and relatively constant throughout the one- and two-cell stages. However, qualitative analysis of the protein synthetic profile on SDS gels has revealed changes which appear around the late one-cell to early two-cell stage. This early change in the pattern of polypeptide synthesis represents the first major qualitative molecular change found so far in development. We present evidence which suggests that the increased synthesis at the early two-cell stage of a small number of polypeptides of molecular weight 35,000 is not dependent on transcription, but rather represents control at a post-transcriptional level using mRNAs synthesised before fertilisation.