A physiological connection between tmRNA and peptidyl-tRNA hydrolase functions in Escherichia coli.
The bacterial ssrA gene codes for a dual function RNA, tmRNA, which possesses tRNA-like and mRNA-like regions. The tmRNA appends an oligopeptide tag to the polypeptide on the P-site tRNA by a trans-translation process that rescues ribosomes stalled on the mRNAs and targets the aberrant protein for degradation. In cells, processing of the stalled ribosomes is also pioneered by drop-off of peptidyl-tRNAs. The ester bond linking the peptide to tRNA is hydrolyzed by peptidyl-tRNA hydrolase (Pth), an essential enzyme, which releases the tRNA and the aberrant peptide. As the trans-translation mechanism utilizes the peptidyl-transferase activity of the stalled ribosomes to free the tRNA (as opposed to peptidyl-tRNA drop-off), the need for Pth to recycle such tRNAs is bypassed. Thus, we hypothesized that tmRNA may rescue a defect in Pth. Here, we show that overexpression of tmRNA rescues the temperature-sensitive phenotype of Escherichia coli (pth(ts)). Conversely, a null mutation in ssrA enhances the temperature-sensitive phenotype of the pth(ts) strain. Consistent with our hypothesis, overexpression of tmRNA results in decreased accumulation of peptidyl-tRNA in E.coli. Furthermore, overproduction of tmRNA in E.coli strains deficient in ribosome recycling factor and/or lacking the release factor 3 enhances the rescue of pth(ts) strains. We discuss the physiological relevance of these observations to highlight a major role of tmRNA in decreasing cellular peptidyl-tRNA load.
Singh
Nongmaithem Sadananda
NS
Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, 560 012, India.
Varshney
Umesh
U
D000483:Alleles; D002265:Carboxylic Ester Hydrolases / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics* / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000235:genetics; D009154:Mutation; D010454:Peptide Termination Factors / Q000235:genetics; D010641:Phenotype; D011401:Promoter Regions, Genetic; D012329:RNA, Bacterial / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism; D013696:Temperature
D029968:Escherichia coli Proteins; D010454:Peptide Termination Factors; D012329:RNA, Bacterial; D012346:RNA, Transfer, Amino Acyl; C489004:frr protein, E coli; C087971:peptide-chain-release factor 3; C019037:tRNA, peptidyl-; C096222:tmRNA; D002265:Carboxylic Ester Hydrolases; C019674:aminoacyl-tRNA hydrolase