Stimulation of Enzymatic Phe-tRNA Binding to Mammalian Ribosomes by Thallium Ions at Concentrations Blocking Other Ribosomal Functions

Abstract

Thallium acetate (TIOAc) effectively stimulates poly(U)-directed Phe-tRNA binding to mouse ascitic tumour ribosomes under conditions when other ribosomal functions are completely blocked. The Tl⁺ optimum is about 200 mM. The reaction is stimulated by EF-1, but not significantly by GTP. EF-1-dependent ribosomal GTPase is inhibited by Tl⁺. The isolated Phe-tRNA ribosome complex is relatively stable. The bound Phe-tRNA does not react with puromycin in the presence of 175 mM KCl. The complex formed in the presence of 90-100 mM TlOAc can, after isolation, be directly utilized for polyphenylalanine synthesis. The complex formed at 200 mM TlOAc is less active, apparently because of damage to the 60-S subunits. TlOAc at low concentrations (8 mM) stimulates K⁺-containing poly(U)-translating system, probably by stabilizing the translation complex.