On the Function of the Ribosomal Protein S1 in the Elongation Cycle of Bacterial Protein Synthesis

The function of the ribosomal protein S1 in the elongation cycle of bacterial protein synthesis was examined. S1 was removed from 30-S ribosomal subunits by adsorption of the subunits to a column of Sepharose 4B linked to anti-S1 IgG, followed by elution of the 30S (−1) subunits with 0.8 M NH4Cl. The depleted 30-S subunits showed, when combined with 50-S subunits, a reduced activity in poly(U)-dependent polyphenylalanine synthesis and in poly(A)-stimulated polylysine synthesis. The ribosomes could be reactivated in these functions by the addition of stoichiometric amounts of S1. The association constants (K1) for the binary complex between 30-S subunit and oligonucleotide, as measured by equilibrium dialysis, revealed no S1 effect. Whereas the activity of 30-S subunits and 70-S ribosomes in the coded binding of Phe-tRNA was influenced by the chain length of the template and the presence of S1, neither effect was found in the oligo(A)-directed binding of Lys-tRNA. Reconstitution of the Phe-tRNA binding capacity of depleted 30-S and 70-S particles by the addition of S1 was dependent on the chain length of the oligouridylate. The association constant (K2) for the binding of Phe-tRNA to the 30-S-subunit · oligonucleotide complex was S1-dependent with oligo(U) as template. It is proposed that S1, in analogy to its postulated function in initiation, binds to the phosphodiester backbone of the mRNA and adjusts the conformation of the codon to optimize double-strand formation with the anticodon of a cgnate tRNA