Elongation factor-dependent affinity labeling of Escherichia coli ribosomes

Abstract

Elongation factor-dependent affinity labeling of Escherichia coli ribosomes was obtained using a functional analogue of aminoacyl-tRNA. Since elongation factor Tu (EF-Tu) screens both the modified aminoacyl-tRNAs and the ribosomal complexes for active particles, only functional macromolecular complexes are examined. This approach also provides an unequivocal identification of the transfer RNA binding site from which affinity labeling occurs. N ε -bromoacetyl-Lys-tRNA was prepared by covalently attaching an electrophilic group to the side-chain of the amino acid. This chemical modification did not interfere with function, since the ϵBrAcLys-tRNA participated successfully in EF-Tu and poly(rA)-dependent binding to ribosomes, peptide bond formation, and elongation factor G (EF-G)-mediated translocation. Affinity labeling of ribosomal RNA was observed only in those incubations which contained both EF-Tu and EF-G. The crosslinking of ϵBrAcLys-tRNA to 23 S rRNA was found even if fusidic acid was added to the incubation before EF-G. The dependence of the covalent reaction on EF-G demonstrates, unambiguously, that a reactive residue of 23 S rRNA is located adjacent to the 3′ end of the functionally defined P site. Similarly, the affinity labeling of proteins L13/14/15, L2, L32/33, and L24 required EF-G-dependent translocation of ϵBrAcLys-tRNA into the P site. Protein L27 was alkylated following the EF-Tu-dependent binding of ϵBrAcLys-tRNA to the ribosome, and the extent of affinity labeling was stimulated by the addition of EF-G to the incubation. Double-label dipeptide experiments confirmed that affinity labeling occurred from functional tRNA binding sites by demonstrating that the same ϵBrAcLys-tRNA which reacted covalently with 23 S rRNA or a ribosomal protein could also participate in peptide bond formation. Finally, the ribosome affinity labeling obtained with ϵBrAcLys-tRNA · EF-Tu · guanylylimidodiphosphate differed little from that obtained with ϵBrAcLys-tRNA · EF-Tu · GTP. This work constitutes the first direct examination of the aminoacyl ends of the EF-Tu-dependent conformational states of the ribosomal complex, and demonstrates the potential value of functional Lys-tRNA analogues with different probes attached to the lysine side-chain.