SmpB triggers GTP hydrolysis of elongation factor Tu on ribosomes by compensating for the lack of codon-anticodon interaction during trans-translation initiation.

Bacterial tmRNA rescues ribosomes that stall because of defective mRNAs via the trans-translation process. Although entry of the charged transfer messenger RNA (tmRNA) into the ribosome proceeded in the absence of elongation factor (EF-Tu) and in the presence of EF-Tu and the antibiotic kirromycin, evidence was found for the involvement of EF-Tu in trans-translation initiation. The polyalanine synthesis system attained by using a tmRNA variant consisting of only the tRNA-like domain revealed that it was completely dependent on the presence of SmpB and greatly enhanced by EF-Tu and EF-G. Actually, ribosome-dependent GTPase activity of EF-Tu was stimulated by the addition of SmpB and tmRNA but independently of template mRNA, demonstrating that SmpB compensates for the lack of codon-anticodon interaction during the first step of the trans-translation initiation. Based on these results, we suggest that SmpB structurally mimics the anticodon arm of tRNA and elicits GTP hydrolysis of EF-Tu upon tmRNA accommodation in the A site of the ribosome.