The Salmonella typhimurium mar locus: molecular and genetic analyses and assessment of its role in virulence

Abstract

The marRAB operon is a regulatory locus that controls multiple drug resistance in Escherichia coli. marA encodes a positive regulator of the antibiotic resistance response, acting by altering the expression of unlinked genes. marR encodes a repressor of marRAB transcription and controls the production of MarA in response to environmental signals. A molecular and genetic study of the homologous operon in Salmonella typhimurium was undertaken, and the role of marA in virulence in a murine model was assessed. Expression of E. coli marA (marA Ec ) present on a multicopy plasmid in S. typhimurium resulted in a multiple antibiotic resistance (Mar) phenotype, suggesting that a similar regulon exists in this organism. A genomic plasmid library containing S. typhimurium chromosomal sequences was introduced into an E. coli strain that was deleted for the mar locus and contained a single-copy marR-lacZ translational fusion. Plasmid clones that contained both S. typhimurium marR (marR St ) and marA (marA St ) genes were identified as those that were capable of repressing expression of the fusion and which resulted in a Mar phenotype. The predicted amino acid sequences of MarR St , MarA St , and MarB St were 91, 86, and 42% identical, respectively, to the same genes from E. coli, while the operator/promoter region of the operon was 86% identical to the same 98-nucleotide-upstream region in E. coli. The marRAB transcriptional start sites for both organisms were determined by primer extension, and a marRAB St transcript of ϳ1.1 kb was identified by Northern blot analysis. Its accumulation was shown to be inducible by sodium salicylate. Open reading frames flanking the marRAB operon were also conserved. An S. typhimurium marA disruption strain was constructed by an allelic exchange method and compared to the wild-type strain for virulence in a murine BALB/c infection model. No effect on virulence was noted. The endogenous S. typhimurium plasmid that is associated with virulence played no role in marA-mediated multiple antibiotic resistance. Taken together, the data show that the S. typhimurium mar locus is structurally and functionally similar to marRAB Ec and that a lesion in marA St has no effect on S. typhimurium virulence for BALB/c mice.

MarA, SoxS and Rob function as virulence factors in an Escherichia coli murine model of ascending pyelonephritis

“…Sulavik et al (1997) Using a murine model of pyelonephritis, we found that E. coli lacking marA, soxS and rob was unable to maintain colonization of the kidney. In this model, the bladders of diuresed mice were infected with bacteria.…”

“…Sulavik et al (1997) used a lethal infection model to investigate the virulence of strains lacking soxS. Van der Straaten and colleagues (2004) failed to find differences during in vivo growth of wild-type organisms compared with strains lacking ramA (specifying another AraC protein), soxS or both genes.…”

Multiple Regulatory Pathways Associated with High-Level Ciprofloxacin and Multidrug Resistance in Salmonella enterica Serovar Enteritidis: Involvement of ramA and Other Global Regulators

“…Mutations within the local repressor AcrR have also been shown to contribute to acrB overexpression (33). The mar locus consists of two transcription units, marC and marRAB, which are divergently transcribed from a central putative operator-promoter region, marO (10,45). MarA regulates its own transcription as well as regulating the expression of the mar regulon, whereas MarR acts by repressing marRAB transcription.…”

“…The functions of MarB and MarC are unknown. marRAB transcription can also be activated by the MarA homologs SoxS and Rob (2,10,45). SoxS is the effector of the soxRS global superoxide response regulon.…”

Bile-Induced DNA Damage in Salmonella enterica

“…1997), and efflux pumps required for bile transport indicated by the observation that Salmonella cultures can outside the cell (Ma et al 1994;Thanassi et al 1997; be adapted to resist extremely high bile concentrations by Nikaido et al 1998). In S. enterica, regulatory functions previous exposure to a sublethal dose (Van Velkinburgh such as MarAB (Sulavik et al 1997) and PhoPQ (Van and Gunn 1999).…”