Bacterial transfer-messenger RNA (tmRNA, 10Sa RNA) is involved in a trans-translation reaction which contributes to the degradation of incompletely synthesized peptides and to the recycling of stalled ribosomes. However, its physiological role in the cell remains elusive. In this study, an efficient system for controlling the expression of the gene for tmRNA (ssrA), as well as a tmRNA gene-defective strain (ssrA::cat), were constructed in Bacillus subtilis. The effects of tmRNA on the growth of the cells were investigated under various physiological culture conditions using these strains.
The cells were viable in the absence of ssrA expression under the usual culture conditions. However, the growth rate of cells without tmRNA expression, relative to that of the expressed cells, decreased with elevating temperature (> 45 °C), and at 52 °C, the highest temperature for growth of the wild-type, cells grew depending on the expression level of tmRNA. Furthermore, the transcription level of the ssrA from the authentic promoter at a high temperature (51 °C) was about 10-fold higher than that at a lower temperature (37 °C). tmRNA-dependent growth and an increase in tmRNA amount were also observed in cells under other stresses, such as high concentrations of ethanol or cadmium chloride. It is also shown that alanylated tmRNA rather than tmRNA-mediated proteolysis is required for growth at high temperature.
The expression of tmRNA gene (ssrA) is required for the efficient growth of B. subtilis under several strong stresses. The transcription of ssrA increases under several stressful conditions, suggesting that it is a stress-response gene. Alanyl-tmRNA, probably via its ability of recycling stalled ribosomes via trans-translation, is involved in the stress tolerance of bacteria.