Inhibitor of protein synthesis co-isolating with polyribosomal RNA

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.