Formylmethionyl‐tRNA Binding Properties of Escherichia coli Ribosomal Protein S1

Abstract

Formylmethionyl‐tRNA has been found to bind to Escherichia coli ribosomal protein S1. Complexes of S1 and methionine‐labeled fMet‐tRNAMetf were detected by binding to cellulose nitrate membranes. Complex formation was stimulated by incubation at low ionic strength, at 37°C and in the absence of Mg2+. Protein S1 also bound [3H]poly(rC) under the same conditions or in buffers of higher ionic strength, 10 mM in Mg2+, and during incubation at 0°C. Unlabeled poly(rC) was an effective inhibitor of the binding of labeled fMet‐tRNA when both nucleotides were added simultaneously to the reaction mixtures; however, approximately 80% of previously bound f[35S]Met‐tRNA remained bound after the subsequent addition of unlabeled poly(rC). Binding of f[35S]Met‐tRNA, but not the binding of [3H]poly(rC), was inhibited by polyamines or aminoglycoside antibiotics. Protein S1 that had been reacted with N‐ethylmaleimide failed to bind f[35S]Met‐tRNA but was still capable of binding [3H]poly(rC).The data support the concept of two distinct RNA‐binding sites on the S1 molecule. Initiator tRNA appears to occupy a site concerned with the RNA unwinding property of S1, whereas poly(rC) binds primarily at a separate site. During initiation on the ribosome, the poly(rC) site may interact with the 3′ end of 16‐S ribosomal RNA, while the other site may interact with either mRNA or fMet‐tRNA.