Summary: Streptomycin dependence can be caused by mutations in ribosomal protein S12. Mutations suppressing such streptomycin dependence have been found in ribosomal proteins S4 and S5, and in 16S rRNA. Here a new suppressor mutation localized in elongation factor Tu (EF-Tu) is described, consistent with recent models of ribosome-EF-Tu-tRNA interaction at the decoding centre. The EF-Tu mutation was obtained by genetic selection for streptomycin independence; it was identified as Ala375 → Thr, previously described as EF-TuA and known to confer a kirromycin-resistant, error-prone phenotype. Also, other streptomycin-dependent (Sm) S12 mutations could be complemented by this mutation. The streptomycin-independent (Sm) strain grows more slowly than the wild-type (wt), suggesting that not all the defects of the S12 mutation can be complemented by EF-Tu[A375T]. Moreover, this strain is more susceptible than wt to reduction in the cellular EF-Tu concentration, and disruption of led to considerable growth-rate impairment. Expression of EF-Tu from , not only of wt EF-Tu and EF-Tu[A375T] but, remarkably, also of EF-Tu[G222D], known as EF-TuB and defective in protein synthesis, equally contributed to cell growth. analysis revealed a decreased translational activity of wt EF-Tu with Sm ribosomes as compared to EF-Tu[A375T], while EF-Tu[G222D] showed no activity at all, just as with wt ribosomes. Possible mechanisms are discussed for the improved growth rate observed in such Sm strains when they include wt EF-Tu or EF-Tu[G222D].


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