ON THE MECHANISM OF RIBOSOMAL ACTIVATION IN NEWLY FERTILIZED SEA URCHIN EGGS

Preformed mRNA and the programming of early embryo development

A significant feature of the early development of fertilized echinoderm and amphibian eggs and germinating seed embryos is the utilization of genetic information that has been previously transcribed during oogenesis and seed ripening. When RNA synthesis is suppressed in the early developing embryos by actinomycin D, cordycepin, or alpha-amanitin, there is no effect on the translation of the "preformed mRNA", only a limited number have been thus far identified; microtubule and histone proteins in the fertilized sea urchin egg and carboxypeptidase and isocritric lyase in germinating cottonseed. Data obtained on the protein synthetic pattern at different times after the onset of development suggest that preformed mRNAs are made available to the translational system in a gradual process, thereby providing a molecular basis for the regulation of development. The possibility is considered that polyadenylation of mRNA, a reaction known to occur early after sea urchin fertilization, is responsible for regulating the release of preformed mRNA. It is shown that this reaction (polyadenylation) can be completely suppressed with little effect on the function of preformed mRNA. Finally, it is suggested, at least for the seed embryo system, that the formation of ATP may be a prerequisite for the activation of protein synthesis.

Reversible degradation of polyribosomes in Chang cells cultured in a glutamine-deficient medium

The effects of temporary glutamine deficiency on the protein and nucleic acid metabolism of Chang's liver cells in suspension cultures have been studied. It was observed that cells maintained in a glutamine-free medium showed a reduced incorporation of labeled precursors into protein and RNA. At the same time, the activity of the ribosomes and the proportion of polyribosomal aggregates in cell extracts diminished. These effects were reversed when the glutamine content of the medium was restored. The restoration of a normal rate of amino acid incorporation by intact cells as well as by cell-free systems was time dependent, and took place within a few hours after glutamine addition without preceding increase in the prevailing low rate of RNA synthesis. The addition of actinomycin D at concentrations that strongly inhibited the RNA metabolism of the cells did not prevent the increase in protein synthesis or the reappearance of polyribosomal aggregates. These facts suggest that the restoration of protein synthesis in the cells after glutamine starvation was not dependent on a production of new messenger RNA. The experimental data are consistent with the hypothesis that previously synthesized messenger RNA, preserved in the cells in a stable form, was brought into action in response to the reestablishment of an adequate cellular environment.

A comparative study in vivo and in vitro of the ability of ribosomes from Xenopus liver and ovary to incorporate L-[U-14C]leucine

1. A system for the incorporation in vitro of amino acids into protein is described for the South African clawed toad (Xenopus laevis laevis Daudin). 2. The incorporation of l-[U-(14)C]leucine by Xenopus-liver microsomes is very much greater per mg. of microsomal RNA than the incorporation by ovary microsomes. 3. The incorporation by Xenopus-liver and -ovary polysomes is approximately the same when expressed per mg. of polysomal RNA. 4. It was predicted from the above results that ovary microsomes should contain a ribosomal fraction inactive in protein synthesis. This was shown to be the case by a labelling experiment in vivo with l-[U-(14)C]leucine. 5. The labelling experiment in vivo also showed that the active polysomal fraction in ovary is associated with membranes and is liberated by treatment with deoxycholate; this is also true of liver microsomes in vivo. 6. The results are discussed in relation to previous work on the synthesis of proteins by amphibian ovarian tissue, and on the role of bound and free ribosomes in protein synthesis.

Factors controlling amino acid incorporation by ribosomes from krebs II mouse ascites-tumour cells

1. A ribosome-cell sap system capable of supporting the incorporation of (14)C-labelled amino acids into protein has been prepared from Krebs II mouse ascites-tumour cells. The requirements of this system for optimum activity and response to added messenger RNA have been investigated. One such system has been obtained for which amino acid incorporation is almost wholly dependent on the addition of suitable messenger RNA. 2. Ribosomes of widely different but predictable activities in the cell-free system have been prepared from Krebs cells pretreated in a variety of ways. The factors in the pretreatment of the cells responsible for these differences have been investigated. 3. The structural and functional properties of these different ribosome preparations and their response to exogenous messenger RNA have been examined and are discussed in the light of modern concepts of the control of protein synthesis.