TorI, a response regulator inhibitor of phage origin in Escherichia coli

doi:10.1073/pnas.0401927101

TorI, a response regulator inhibitor of phage origin in Escherichia coli

Laboratoire de Chimie Bactérienne, Institut de Biologie Structurale et Microbiologie, Centre National de la Recherche Scientifique, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France

Edited by Susan Gottesman, National Institutes of Health, Bethesda, MD (received for review March 18, 2004)

Abstract

The torI gene has been identified by using a genetic multicopy approach as a negative regulator of the torCAD operon that encodes the trimethylamine N-oxide reductase respiratory system in Escherichia coli. The negative effect was due to a previously unidentified small ORF (66 aa) of phage origin that we called torI for Tor inhibition. Overexpression of torI led to an 8-fold decrease of the torCAD operon transcription. This operon is positively regulated, in the presence of trimethylamine N-oxide, by a four-step phosphorelay involving the TorS sensor and the TorR response regulator. Epistatic experiments showed that TorI acts downstream of TorS and needs the presence of TorR. In vitro experiments showed that it is neither a TorR phosphatase nor a histidine kinase inhibitor and that it binds to the effector domain of TorR. Unexpectedly, TorI did not impede TorR DNA binding, and we propose that it may prevent RNA polymerase recruitment to the torC promoter. This study thus reveals a previously uncharacterized class of response regulator inhibitors.

Footnotes

↵ * To whom correspondence should be addressed. E-mail: ansaldi{at}ibsm.cnrs-mrs.fr.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: TMAO, trimethylamine N-oxide; IPTG, isopropyl β-d-thiogalactoside; BMH, bismaleimidohexane.