Oversynthesis of a new Escherichia coli small RNA suppresses export toxicity of DsbA '-PhoA unfoldable periplasmic proteins

In Escherichia coli, the DsbA'-PhoA hybrid proteins carrying an unfoldable DsbA' fragment can be targeted to the envelope, where they exert their toxi city. Hybrid proteins stick to the periplasmic face of the inner membrane a nd paralyze the export mechanism, becoming lethal if sufficiently overprodu ced and if not degraded by the DegP protease (A. Guigueno, P. Belin, and P. L. Boquet, J. Bacteriol. 1.79:3260-3269, 1997). We isolated a multicopy su ppressor that restores viability to a degP strain without modifying the exp ression level of the toxic fusion. Suppression does not involve activation of the known envelope stress-combative pathways, the Cpx pathway and the si gma (E) regulon. Subclone analysis of the suppressor revealed a 195-bp DNA fragment that is responsible for toxicity suppression. The cloned gene, tai led uptR, is approximate to 130 bp long (including the promoter and a trans cription termination signal) and is transcribed into a small RNA (92 nucleo tides). Using site-directed mutagenesis, we found that UptR RNA does not re quire translation for toxicity suppression. UptR-mediated action reduces th e amount of membrane-bound toxic hybrid protein. UptR RNA is the first exam ple of a small RNA implicated in extracytoplasmic toxicity suppression. It appears to offer a new way of suppressing toxicity, and its possible modes of action are discussed.