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SummaryA characterization of the LtrR regulator, an S. Typhi protein belonging to the LysR family is presented. Proteomics, outer membrane protein profiles and transcriptional analyses demonstrated that LtrR is required for the synthesis of OmpR, OmpC and OmpF. DNA-protein interaction analysis showed that LtrR binds to the regulatory region of ompR and then OmpR interacts with the ompC and ompF promoters inducing porin synthesis. LtrR-dependent and independent ompR promoters were identified, and both promoters are involved in the synthesis of OmpR for OmpC and OmpF production. To define the functional role of the ltrR-ompR-ompC-ompF genetic network, mutants in each gene were obtained. We found that ltrR, ompR, ompC and ompF were involved in the control of bacterial transformation, while the two regulators and ompC are necessary for the optimal growth of S. Typhi in the presence of one of the major bile salts found in the gut, sodium deoxycholate. The data presented establish the pivotal role of LtrR in the regulatory network of porin synthesis and reveal new genetic strategies of survival and cellular adaptation to the environment used by Salmonella.

OmpW of Salmonella enterica serovar Typhimurium has been described as a minor porin involved in osmoregulation, and is also affected by environmental conditions. Biochemical and genetic evidence from our laboratory indicates that OmpW is involved in efflux of and resistance towards paraquat (PQ), and its expression has been shown to be activated in response to oxidative stress. In this study we have explored ompW expression in response to PQ. Primer extension and transcriptional fusions showed that its expression was induced in the presence of PQ. In silico analyses suggested a putative binding site for the SoxS transcriptional factor at the ompW regulatory region. Electrophoretic mobility shift assays (EMSAs) and footprinting experiments showed that SoxS binds at a region that starts close to −54 and ends at about −197 upstream of the transcription start site. Transcriptional fusions support the relevance of this region in ompW activation. The SoxS site is in the forward orientation and its location suggests that the ompW gene has a class I SoxS-dependent promoter.

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