Temperature-responsive regulation of the Escherichia coli lpxT gene

RNA thermometers (RNATs) are thermo-labile mRNA secondary structures modulating translation initiation in response to temperature changes. We found that the expression of Pseudomonas aeruginosa LpxT, a membrane protein phosphorylating the lipid A moiety of the lipopolysaccharide, was induced in response to a temperature upshift and we proposed that an RNAT was responsible for such regulation. Here we show that the expression of the Escherichia coli LpxT orthologous protein (EcLpxT) is also temperature-responsive. Increased EcLpxT expression at high temperature (i.e. 37-42 vs. 28°C) depends on post-transcriptional mechanisms. In vitro assembly of the ribosome on the EclpxT transcript is less efficient at 28° than at 37-42°C. The short lpxT 5’-UTR is predicted to form an unstable stem-loop involving the Shine-Dalgarno region (SD). Point mutations changing the stability of the stem-loop or ameliorating the complementarity of the SD with the 16S rRNA affect thermoregulation, showing that both elements cooperate in lpxT gene regulation. Our results suggest that Ec lpxT is regulated by an unusual RNAT that exploits the combination of sub-optimal elements to confer temperature-responsive translation to the mRNA.