Mechanisms of ribosome rescue in bacteria.

Ribosomes that stall during translation need to be rescued to ensure that the protein synthesis capacity of the cell is maintained. Stalling arises when ribosomes become trapped at the 3' end of an mRNA, which occurs when a codon is unavailable, as this leads to the arrest of elongation or termination. In addition, various factors can induce ribosome stalling in the middle of an mRNA, including the presence of specific amino acid sequence motifs in the nascent polypeptide. Almost all bacteria use a mechanism known as trans-translation to rescue stalled ribosomes, and some species also have other rescue mechanisms that are mediated either by the alternative ribosome-rescue factor A (ArfA) or ArfB. In this Review, I summarize the recent studies that have demonstrated the conditions that trigger ribosome stalling, the pathways that bacteria use to rescue stalled ribosomes and the physiological effects of these processes.