Poly(A) RNA of the egg cytoplasm: structural resemblance to the nuclear RNA of somatic cells

This paper concerns the structural characteristics of the poly(A) RNA stored in unfertilized amphibian and echinoderm eggs. Though located in the egg cytoplasm, at least two-thirds of these maternal transcripts display an interspersed sequence organization similar to that of nuclear RNA. In Xenopus laevis interspersed poly(A) RNA molecules are synthesized and deposited in the oocyte cytoplasm throughout the main growth phase of oogenesis. Regions of the sea urchin genome that are represented by interspersed maternal transcripts have been recovered from recombinant clone libraries. In one case the same single-copy sequence is found both in an abundant message-sized 1.6 kilobase (kb) maternal transcript and in a 7.5 kb maternal transcript that structurally resembles a precursor form and is not found in embryonic polysomes. In a second example considered, a 9.5 kb transcript was identified in embryo nuclear RNA that may be identical in structure with an interspersed maternal poly(A) RNA derived from the same transcription unit. Transcription of this sequence appears to be constitutive in somatic cell nuclei, though no homologous cytoplasmic RNAs are found after early cleavage. This may be a widespread form of regulation for transcription units expressed in female germ cells, and represented in the maternal poly(A) RNA pools of unfertilized eggs.