Translational control of protein synthesis in stimulated WI-38 fibroblasts

High salt effect on RNA synthesis in Krebs ascites tumor cells

The incubation of Krebs ascites tumor cells in medium with a high salt concentration resulted in a partial inhibition of nuclear RNA synthesis. The residual RNA polymerase activity in such nuclei was only slightly inhibited by low concentrations (50 nM) of alpha-amanitin. This finding suggested an inhibition of RNA polymerase II activity under conditions of medium hypertonicity. Indeed, RNA polymerase II, isolated from the nuclei of cells exposed to hypertonicity, revealed only half of the control activity. On the other hand, RNA polymerase I was not affected by hypertonicity. Moreover, chromatin fractions isolated from cells incubated in hypertonic or isotonic medium were equally template-active in RNA synthesis.

Regulation of the peptide elongation reaction on uterine ribosomes by estrogens

Administration of 17 beta-estradiol to ovariectomized mature rats for 1 h induces an increased capacity of subsequently isolated uterine ribosomes to synthesize protein in a cell-free protein synthesis system. The increased rate of protein synthesis can be ascribed to an effect of estrogen on the rate of peptide elongation rather than synthesis of additional new peptides. The increased rate of peptide elongation is dependent upon the dose of estradiol over the range of 0.1 to 10 micrograms/animal, and exhibits hormone specificity; 17 beta-estradiol, diethylstilbesterol, estrone and estriol but not 17 alpha-estradiol, progesterone, dihydrotestosterone or corticosterone will induce the response. Removal of ribosome associated proteins by extraction with 0.5 M KCl results in activation of protein synthesis by uterine ribosomes from control rats to rates that are equal to that of ribosomes from estrogen-stimulated rats suggesting that ribosomes from control animals are in an inhibited state. The KCI extracted ribosomal factors from control animals inhibit the synthesis of protein by salt-washed uterine ribosomes when added back to the ribosomes prior to assay and the inhibitory properties of these factors are greater if derived from ribosomes of control rather than 1 h estradiol-treated rats. The extracted inhibitor is inactivated by heat, is insensitive to treatment with N-ethylmaleimide, is insensitive to micrococcal nuclease and is reversible. The early activation of uterine ribosomes by estrogen appears to result from either the removal or inactivation of a ribosome associated-peptide elongation reaction, inhibitory factor.

The effect of cessation of growth on protein synthesis in a mutant of Chinese hamster ovary cells with a temperature-sensitive leucyl-tRNA synthetase

A temperature-sensitive mutant of Chinese hamster ovary cells with an altered leucyl-tRNA synthetase fails to grow and to incorporate amino acids into protein properly at or near the non-permissive temperature. This mutant was used to determine whether cessation of growth at the elevated temperature affected elongation factor EF-1, since the activity of EF-1 is markedly lower in non-growing cells in stationary phase than in rapidly-growing cells in exponential phase. Cell-free extracts prepared from cells maintained at 39 degrees C for 24 h showed a marked decrease in the ability to translate natural mRNAs, compared to cells incubated at 34 degrees C. However, the ability to translate poly(U), which requires elongation factor EF-1 (and EF-2), was not affected. Analyses of activities involved in the initiation of protein synthesis and in the activation of amino acids revealed that, with the exception of leucyl-tRNA synthetase, the rest of the components required for translation also appeared to be relatively stable even after 24 h at the elevated temperature. The effects of elevated temperature on cell-free extracts were also investigated. The results were similar to those obtained with intact cells; that is, except for leucyl-tRNA synthetase which was rapidly inactivated in vitro at 39 degrees C, other aminoacyl-tRNA synthetases and translational components involved in chain initiation and elongation were relatively stable. Thus, no change in EF-1 activity was detected as a result of arrested cell growth, an inherent lability of the elongation factor, or metabolic degradation as a consequence of a rapid turnover rate in the absence of protein synthesis.

An inhibitor which interferes with the enzymatic aminoacylation of tRNA

Inactive, frozen and thawed cytoplasmic extracts of 3T3 and SV-101 (3T3 transformed by SV-40 virus) cells contain an inhibitor which blocks the poly(U)-directed incorporation of [14C]phenylalanine into polypeptides, catalyzed by active extracts of these cells. This inhibition is not reversed by adding increased amounts of poly(U). Furthermore, little or no inhibitory activity is observed when poly(U) translation is assayed using precharged [14C]Phe-tRNA. These results suggest that the observed inhibition is not due to the degradation of poly(U) by a nuclease. The inhibitor appears to act primarily at the level of tRNA charging since the synthesis of both Phe-tRNA and Lys-tRNA is impaired in its presence. Evidence is presented which indicates that the inhibitory activity is not due to a high molecular weight protein or nucleic acid. However, the inhibitor appears to be adsorbed to a macromolecule. The inhibitory activity is completely destroyed by ashing.

Studies of the protein synthetic activity of lysates from HeLa cells incubated in hypertonic medium

When HeLa S-3 cells are incubated with medium made hypertonic by adding NaCl, protein synthesis is inhibited. When the cells are returned to normal conditions protein synthesis is restored. To study the molecular mechanism of this regulation of protein synthesis, lysates were prepared from HeLa S-3 cells grown in minimum essential medium (normal, N); from cells which were incubated with additional (100 mM) NaCl (hypertonic, H); and from cells which were treated similarly in hypertonic medium and then restored to isotonic conditions (hypertonic-isotonic, H-I). Lysates of H cells exhibited reduced endogenous protein synthesis. Studies with mixed lysates from H and N cells implicated that the H lysate did not contain a soluble, non-labile macromolecule (greater than 10 000 daltons) with an inhibitory activity upon the protein synthesis. Analysis by Edman reaction of H lysates showed a reduced incorporation of [35S]methionine into N-terminal suggesting that the initiation step of protein synthesis was affected. However, sucrose gradietn analysis indicated that lysates of H cells were still able to form 80-S initiation complexes with [35S] methionyl-tRNAIMet. The block in initiation was not complete. The lesion could not be reversed by adding post-ribosomal supernatant or a ribosomal salt wash from N cells to ribosomes from the H cells. The data show that the ribosomal fraction is primarily involved in the inhibition.