Resuming translation on tmRNA: a unique mode of determining a reading frame.

The bacterial ribosome switches from an mRNA lacking an in-frame stop codon and resumes translation on a specialized RNA known as tmRNA, SsrA or 10Sa RNA. We find that the ribosome can reach and use the extreme 3' terminal codon of the defective mRNA prior to switching. The first triplet to be translated in tmRNA (the resume codon) is determined at two levels: distant elements in tmRNA restrict resume codon choice to a narrow window and local upstream elements provide precision. Insights from a randomization-selection experiment secure the alignment of tmRNA sequences from diverse species. The triplet UA(A/G) (normally recognized as a stop codon by release factor-1) is strongly conserved two nucleotides upstream of the resume codon. The central adenosine of this triplet is essential for tmRNA activity. The reading frame of tmRNA is determined differently from all other known reading frames in that the first translated codon is not specified by a particular tRNA anticodon.