Studies on 30S Ribosomal Protein S1 from E. coli

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Abstract
The functional properties of purified ribosomal protein SI were studied in a cell-free system for the translation of synthetic mRNA, and the following results were obtained. 1. Protein SI was required for the translation of poly(U) in the presence of purified elongation factors, labeled Phe-tRNA, and 70S ribosomes from which protein SI had been removed by means of a poly(U)-cellulose column. 2. In contrast, poly(A)-directed polylysine synthesis in a similar system was not dependent on added protein SI. 3. The binding of poly(U) was greatly enhanced in Sl-enriched ribosomes as compared to Si-depleted ribosomes. On the other hand, the ability of ribosomes to bind poly(A) was not much impaired by removal of protein SI. 4. The binding of poly(U) to Sl-enriched or Sl-depleted ribosomes was markedly stabilized by the addition of unacylated tRNAPhe. Unacylated tRNA for phenylalanine but not that for glutamic acid or isoleucine could stimulate poly(U) translation by the Sl-depleted ribosomes. Sl-enriched ribosomes were active in the absence of unacylated tRNAPhe, but were further stimulated by its addition. 5. IF-3 was found to shift the optimum concentration of Mg2+ for the translation of poly(U) and poly(A). At higher concentrations of Mg2+ (about 12 mM), the translation became dependent on IF-3, whereas at lower Mg2+ levels (about 5 mM) the presence of IF-3 was rather inhibitory.

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