In order to examine the nature of the complex formation between the ribosomal protein S1 and nucleic acids three methods were used: Inhibition of the reaction of n-ethyl[2.3 14C]-maleimide with S1 by the addition of oligonucleotides; adsorption of the complexes to nitrocellulose filters; and equilibrium dialysis. The complex formation is Mg2+ dependent at low salt concentrations and becomes Mg2+ independent at an ionic strength greater than 90 mM. Oligouridylates of increasing chain length reach an optimal KA of 3-3-10(7) M-1 at a chain length of n=13-14. Protein S1 contains one binding site for long chain oligouridylates, such as U12, and the standard-free-energy change on binding caused by one Pu increment is 0.41 kcal/mol, when n varies between five and fourteen. Complex formation is insensitive to the capacity of the homopolynucleotide bases to form hydrogen bonds. Homopolynuceotides, however, showing a Tm less than 250 in the buffer system used show an increased affinity for S1 compared to poly(A) and poly(C) (Tm greater than 40 degrees). The data are discussed with respect to the proposed binding of protein S1 to the 3-terminal end of the 16S RNA.

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 NH4C1. 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 (Kl) 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 (&) 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 cognate tRNAThe protein S1, a 30-S ribosomal subunit constituent, is required for the translation of natural messengers like phage MS2 RNA or R17 RNA [1,2].Furthermore it is one of the subunits of Qp replicase involved in the selective copying of the plus strand of

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