A molecular approach to the problem of positional information in eggs and early embryos

It is proposed that long maternal RNA transcripts, which are known not to be translated, have a positive role in defining positional information in the egg or early embryo of invertebrates and vertebrates. The sequences of these RNAs can, in principle, provide a large store of morphogenetic information in two and three dimensions specified by the primary sequence in DNA. This could occur by complementary base pairing within and between molecules to build up in stages a large three-dimensional scaffold or cytoskeleton stabilized by proteins. This would define specific regions or compartments in the egg, and embryonic nuclei would be distributed to different cytoplasmic environments. The RNA and associated proteins would provide the basis for specific interactions with nuclear DNA, including the transport of DNA binding proteins to the nucleus and the activation of latent mRNA molecules associated with the RNA-protein scaffold. Such interactions would produce the specific gene expression required for further embryonic development. Some features of early development of a few well studied species are briefly discussed in relation to the proposed mechanism for RNA specification in space.