A-Site mRNA Cleavage Is Not Required for tmRNA-Mediated ssrA-Peptide Tagging

• DOI: 10.1371/journal.pone.0081319
• Publication Date: 2013-11-19T22:16:18Z
• Authors: Brian D. Janssen; Fernando Garza-Sá...; Christopher S. Hayes
• Keywords: RNA Cleavage; 0301 basic medicine; Science; RNA Stability; Q; R; RNA-Binding Proteins; Gene Expression Regulation, Bacterial; 12. Responsible consumption; RNA, Bacterial; 03 medical and health sciences; Protein Biosynthesis; Codon, Terminator; Escherichia coli; Medicine; Ribosomes; Research Article

In Escherichia coli, prolonged translational arrest allows mRNA degradation into the A site of stalled ribosomes. The enzyme that cleaves the A-site codon is not known, but its activity requires RNase II to degrade mRNA downstream of the ribosome. This A-site mRNA cleavage process is thought to function in translation quality control because stalled ribosomes are recycled from A-site truncated transcripts by the tmRNA-SmpB "ribosome rescue" system. During rescue, the tmRNA-encoded ssrA peptide is added to the nascent chain, thereby targeting the tagged protein for degradation after release from the ribosome. Here, we examine the influence of A-site mRNA cleavage upon tmRNA-SmpB activity. Using a model transcript that undergoes stop-codon cleavage in response to inefficient translation termination, we quantify ssrA-peptide tagging of the encoded protein in cells that contain (rnb(+)) or lack (Δrnb) RNase II. A-site mRNA cleavage is reduced approximately three-fold in Δrnb backgrounds, but the efficiency of ssrA-tagging is identical to that of rnb(+) cells. Additionally, pulse-chase analysis demonstrates that paused ribosomes recycle from the test transcripts at similar rates in rnb(+) and Δrnb cells. Together, these results indicate that A-site truncated transcripts are not required for tmRNA-SmpB-mediated ribosome rescue and suggest that A-site mRNA cleavage process may play a role in other recycling pathways.