Isolation and characterization of an inhibitor of ribosome-dependent GTP hydrolysis by elongation factor G

Regulation of the uncoupled GTPase activity of elongation factor G (EF-G) by the conformations of the ribosomal subunits

The elongation factor G (EF-G) GTPase activity is induced by either 70S ribosomes or 50S ribosomal subunits. The GTPase activity induced by 50S ribosomal subunits is predominant at low concentrations of monovalent cations and decreases with increasing concentrations of K+ or NH4+. Double-logarithmic plots of the data reveal straight lines with different slopes for low and high concentrations of monovalent cations, respectively, intersecting at the same concentration of monovalent cations where maximal EF-G GTPase activity is measured in the presence of both ribosomal subunits. Substantially the same curves are obtained when 50S ribosomal subunits are substituted by 50S CsCl-core particles partially reconstituted by addition of purified 50S split proteins L7/L12. Intact 30S ribosomal subunits, but not 30S CsCl-core particles are able to associate with 50S ribosomal subunits and to modulate ribosome-dependent EF-G GTPase activity. Therefore, our data clearly show that the biphasic courses of the NH4+ and K+ curves of EF-G GTPase activity induced by 50S ribosomal subunits are not due to contaminations with 30S ribosomal subunits but result from different conformations of EF-G/50S ribosomal-subunit complexes at low and high concentrations of monovalent cations, respectively. CD spectra of 50S ribosomal subunits measured under different salt conditions have shown that the conformation of the 50S ribosomal subunits is strongly dependent on the concentration of monovalent cations. The conformation of 30S ribosomal subunits is, however, considerably stronger influenced by the Mg2+ than by the concentration of monovalent cations. The salt effects on the conformation of the 30S ribosomal subunits correspond to the salt effects on the association of ribosomal subunits and the modulation of EF-G GTPase activity by 30S ribosomal subunits. Since, in the presence of both ribosomal subunits, EF-G GTPase activity is maximal at the same concentration of monovalent cations where obviously a spontaneous conformation change of 50S ribosomal subunits takes place, we postulate that EF-G GTPase primarily acts on the ribosomes by changing the conformation of 50S ribosomal subunits. The resulting model is based on the assumption that EF-G GTPase activity is considerably more strongly induced by the 'substrate conformation' ('state I') than by the 'product conformation' of the 50S ribosomal subunits ('state II'). A spontaneous transformation of 'state II' to 'state I' is expected to occur in the absence of mRNA, aminoacyl-tRNA and EF-T especially under salt conditions favouring state I.

An inhibitor of elongation factor G (EF-G) GTPase present in the ribosome wash of Escherichia coli: a complex of initiation factors IF1 and IF3?

An inhibitor of elongation factor G (EF-G) GTPase isolated from the ribosome wash of Escherichia coli was shown to stimulate the poly(A,U,G)- and initiation factor 2 (IF2)-dependent binding of N-formyl-[35S]Met-tRNAfMet to ribosomes. In the presence of saturating amounts of the EF-G GTPase inhibitor, neither addition of initiation factor 1 (IF1) nor addition of initiation factor 3 (IF3) caused a further stimulation of the formation of N-formyl-[35S]Met-tRNAfMET/poly(A,U,G)/ribosome complexes. Both IF1 and IF3 were shown to inhibit ribosome-dependent EF-G GTPase, especially when both initiation factors were added either in absence or in the presence of initiation factor 2 (IF2), poly(A,U,G) and N-formyl-Met-tRNAfMet. Therefore, we conclude that the EF-G GTPase inhibitor consisting of two polypeptide subunits with apparent molecular masses of 23,000 and 10,000 Da is a complex of initiation factors IF1 and IF3. The inhibition of EF-G GTPAse by IF3, but not the effects of IF1 in the presence or absence of IF3 could be reversed by increasing the Mg(2+)-concentration as already shown for the EF-G GTPase inhibitor. Therefore, IF1 as well as the EF-G GTPase inhibitor do not influence the ribosome-dependent EF-G GTPase by affecting the association of ribosomal subunits.