Bacterial SsrA system plays a role in coping with unwanted translational readthrough caused by suppressor tRNAs

Bacterial SsrA RNA (also known as tmRNA or 10Sa RNA) mediates the addition of a short peptide tag to the C-terminus of the nascent polypeptide when a ribosome is stalled at the 3' end of an mRNA lacking a stop codon. This process, called trans-translation, rescues the stalled ribosome and ensures degradation of tagged polypeptides by ATP-dependent proteases. To fully understand the physiological roles of SsrA RNA, it is essential to know how endogenous mRNA targets for the SsrA system are generated in cells. The aim of the present study is to examine how translational readthrough by suppressor tRNAs affects trans-translation in Escherichia coli.

We demonstrated that SsrA tagging of bulk cellular proteins was significantly enhanced by an ochre or an amber suppressor tRNA. Western blot analysis of proteins produced from specific genes possessing a Rho-independent terminator revealed that readthrough at the normal stop codon leads to an efficient tagging and proteolysis of the extended proteins. Size analyses of both protein and mRNA suggested that tagging of extended proteins occurs because ribosome passing through the normal stop codon presumably reach the 3' end of mRNA defined by the transcription terminator hairpin. The inhibitory effect of ssrA mutation on cell growth was markedly amplified in cells with an ochre suppressor tRNA.

The present finding suggests that the SsrA system contributes to scavenge errors and/or problems caused by translational readthrough that occurs typically in the presence of a suppressor tRNA.