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 co don of the defective mRNA prior to switching. The first triplet to be trans lated in tmRNA (the resume codon) is determined at two levels: distant elem ents in tmRNA restrict resume codon choice to a narrow window and local ups tream elements provide precision. Insights from a randomization-selection e xperiment 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 cen tral adenosine of this triplet is essential for tmRNA activity. The reading frame of tmRNA is determined differently from all other known reading fram es in that the first translated codon is not specified by a particular tRNA anticodon.