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J. Bacteriol., 03 1997, 1857-1866, Vol 179, No. 6
MC Sulavik, M Dazer and PF Miller
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 (marAEC) 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 (marRSt) and marA (marASt) 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 MarRSt, MarASt, and MarBSt
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 marRABSt transcript of approximately 1.1 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 marRABEc and that a lesion in marASt has no effect
on S. typhimurium virulence for BALB/c mice.
Copyright © 1997, American Society for Microbiology
The Salmonella typhimurium mar locus: molecular and genetic analyses and assessment of its role in virulence
Therapeutics Department, Parke-Davis Pharmaceutical Research, Division of Warner-Lambert Co., Ann Arbor, MI 48106-1047, USA.
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