Published online 16 November 2004
Nucleic Acids Research, Vol. 32 No. 20 © Oxford University Press 2004; all rights reserved
A physiological connection between tmRNA and peptidyl-tRNA hydrolase functions in Escherichia coli
Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, 560 012, India
* To whom correspondence should be addressed. Tel: +91 80 2293 2686; Fax: +91 80 2360 2697; Email: varshney{at}mcbl.iisc.ernet.in
Received August 6, 2004; Revised October 3, 2004; Accepted October 18, 2004
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 (pthts). Conversely, a null mutation in ssrA enhances the temperature-sensitive phenotype of the pthts 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 pthts strains. We discuss the physiological relevance of these observations to highlight a major role of tmRNA in decreasing cellular peptidyl-tRNA load.
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