Occasionally, ribosomes stall on mRNAs prior to the completion of the polypeptide chain. In Escherichia coli and other eubacteria, tmRNA-mediated trans-translation is a major mechanism that recycles the stalled ribosomes. The tmRNA possesses a tRNA-like domain and a short mRNA region encoding a short peptide (ANDENYALAA in E. coli) followed by a termination codon. The first amino acid (Ala) of this peptide encoded by the resume codon (GCN) is highly conserved in tmRNAs in different species. However, reasons for the high evolutionary conservation of the resume codon identity have remained unclear. In this study, we show that changing the E. coli tmRNA resume codon to other efficiently translatable codons retains efficient functioning of the tmRNA. However, when the resume codon was replaced with the low-usage codons, its function was adversely affected. Interestingly, expression of tRNAs decoding the low-usage codon from plasmid-borne gene copies restored efficient utilization of tmRNA. We discuss why in E. coli, the GCA (Ala) is one of the best codons and why all codons in the short mRNA of the tmRNA are decoded by the abundant tRNAs.Occasionally, during the process of translation, ribosomes stall on mRNAs prior to the completion of the polypeptide chain. If not recycled, accumulation of the stalled ribosomes is detrimental to the organism as it sequesters not only the ribosomes but also the tRNAs (as peptidyl-tRNAs) and brings protein synthesis to a halt. In Escherichia coli and other eubacteria, a well-known pathway that recycles the stalled ribosomes utilizes tmRNA (SsrA or 10Sa RNA) by a process called trans-translation (18,36). The tmRNA contains a tRNA like domain and an mRNA region encoding a short peptide (AN DENYALAA in E. coli) followed by a termination codon (Fig. 1A). The tmRNA is aminoacylated with alanine (in its tRNAlike domain) and recruited to the A-site of the ribosome stalled on a truncated mRNA. The polypeptide from the P-site tRNA is transferred to the alanine on the tRNA-like domain of tm-RNA to form (peptidyl)-alanyl-tmRNA. The ribosome then resumes translation (trans-translation) decoding the first codon (the "resume codon") and the remaining codons of the short open reading frame in the tmRNA. When the termination codon in the tmRNA reaches the ribosomal A-site, the C-terminally tagged polypeptide is released. Thus, the process of trans-translation in E. coli appends AANDENYALAA sequence to the incomplete polypeptide and marks it for degradation by cellular proteases. The C-terminal amino acids in the appended sequence are important for recognition by the cellular proteases; and while the tmRNA mutants that encode ANDENYALDD or ANDEHHHHHH rescue the stalled ribosomes, they do not direct the tagged proteins to degradation (9,11,17,25). Interestingly, the first amino acid (Ala) of the short peptide encoded by the resume codon GCN is evolutionarily conserved across the tmRNA sequences from different species. In addition, although the ssrA mutants harboring alternate resume codons have been...