trans-translation-mediated tight regulation of the expression of the alternative ribosome-rescue factor ArfA in Escherichia coli.

Ribosomes translating mRNA without an in-frame stop codon (non-stop mRNA) stall at its 3' end. In eubacteria, such ribosomes are rescued by SsrA-mediated trans-translation. Recently, we have shown that Escherichia coli ArfA (formerly YhdL) also rescues stalled ribosomes by a mechanism distinct from that of trans-translation. Synthetic lethality phenotype of ssrA arfA double mutants suggests that accumulation of stalled ribosomes is deleterious to E. coli cells. In this report, we show that the expression of ArfA is tightly regulated by the system involving trans-translation. Both premature transcription termination and specific cleavage by RNase III were programmed at the specific sites within the arfA open reading frame (ORF) and produced arfA non-stop mRNA. C-terminally truncated ArfA protein synthesized from arfA non-stop mRNA was tagged through SsrA-mediated trans-translation and degraded in wild type cell. In the absence of SsrA, however, C-terminally truncated ArfA escaped from degradation and had a function to rescue stalled ribosomes. Full-length ArfA produced only when arfA mRNA escapes from both premature transcription termination and RNase III cleavage was unstable. From these results, we illustrate a regulatory model in which ArfA is expressed only when it is needed, namely, when the ribosome rescue activity of trans-translation system is insufficient to support cell viability. This sophisticated regulatory mechanism suggests that the ArfA-mediated ribosome rescue is a backup system for trans-translation.