RNA and protein synthesis in the unfertilized eggs of Urechis caupo

Abstract

Mature, unfertilized eggs of Urechis caupo which are in the prophase of the first meiotic division and contain an intact germinal vesicle synthesize both RNA and protein. Incorporation of radioactive precursors into these macromolecules continues during very long (up to 2 months) incubations in seawater. Autoradiograms demonstrate that all eggs are engaged in synthetic activity. Amino acid label incorporated into protein appears throughout the nucleus and cytoplasm. RNA label, however, is predominantly nuclear and is particularly heavy over the nucleolus, even after several days' incubation with uridine- 3H. The RNA synthesized by the unfertilized eggs includes ribosomal and transfer RNA's plus a third class of RNA which sediments heterogeneously in sucrose gradients, has a more DNA-like base composition, and includes rapidly sedimenting species. Although the sedimentation profiles and labeling kinetics of RNA extracted from these eggs indicate active ribosomal RNA synthesis, only a small portion of the radioactivity is associated with 80 S ribosomes, even after prolonged incubations. The heavier species of radioactive RNA extracted from ribosomes sediments at 28 S, like the bulk RNA, but a radioactivity peak at 30 S predominates in other fractions of egg homogenates, as well as in the total egg RNA extracted directly with phenol-SDS. The predominance of 30 S RNA may be related to the fact that most of the labeled RNA is retained within the nuclei. The RNA and protein synthesis in unfertilized Urechis eggs is not triggered by exposure to seawater, but can be demonstrated in coelomic fluid as well. Immature oocytes of a variety of sizes are active in RNA and protein synthesis when incubated in seawater with no follicle or other accessory cells present. Ribosomal RNA synthesis is active in immature oocytes labeled in seawater or in vivo.