Distinct tmRNA sequence elements facilitate RNase R engagement on rescued ribosomes for selective nonstop mRNA decay

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trans-Translation, orchestrated by SmpB and tmRNA, is the principal eubacterial pathway for resolving stalled translation complexes. RNase R, the leading nonstop mRNA surveillance factor, is recruited to stalled ribosomes in atrans-translation dependent process. To elucidate the contributions of SmpB and tmRNA to RNase R recruitment, we evaluatedEscherichia coli–Francisella tularensischimeric variants of tmRNA and SmpB. This evaluation showed that while the hybrid tmRNA supported nascent polypeptide tagging and ribosome rescue, it suffered defects in facilitating RNase R recruitment to stalled ribosomes. To gain further insights, we used established tmRNA and SmpB variants that impact distinct stages of thetrans-translation process. Analysis of select tmRNA variants revealed that the sequence composition and positioning of the ultimate and penultimate codons of the tmRNA ORF play a crucial role in recruiting RNase R to rescued ribosomes. Evaluation of defined SmpB C-terminal tail variants highlighted the importance of establishing the tmRNA reading frame, and provided valuable clues into the timing of RNase R recruitment to rescued ribosomes. Taken together, these studies demonstrate that productive RNase R-ribosomes engagement requires activetrans-translation, and suggest that RNase R captures the emerging nonstop mRNA at an early stage after establishment of the tmRNA ORF as the surrogate mRNA template.

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