L. Gambino et al., OVEREXPRESSION OF THE MARA POSITIVE REGULATOR IS SUFFICIENT TO CONFERMULTIPLE ANTIBIOTIC-RESISTANCE IN ESCHERICHIA-COLI, Journal of bacteriology, 175(10), 1993, pp. 2888-2894
A genetic approach was undertaken to identify normal bacterial genes w
hose products function to limit the effective concentration of antibio
tics. In this approach, a multicopy plasmid library containing cloned
Escherichia coli chromosomal sequences was screened for transformants
that showed increased resistance to a number of unrelated antibiotics.
Three such plasmids were identified, and all contained sequences orig
inating from the mar locus. DNA sequence analysis of the minimal compl
ementation unit revealed that the resistance phenotype was associated
with the presence of the marA gene on the plasmids. The putative marA
gene product is predicted to contain a helix-turn-helix DNA binding do
main that is very similar to analogous domains found in three other E.
coli proteins. One such similarity was to the SoxS gene product, the
elevated expression of which has previously been associated with the m
ultiple antibiotic resistance (Mar) phenotype. Constitutive expression
of marA conferred antibiotic resistance even in cells carrying a dele
tion of the chromosomal mar locus. We have also found that transforman
ts bearing marA plasmids show a significant reduction in ompF translat
ion but not transcription, similar to previously described mar mutants
. However, this reduction in ompF expression plays only a minor role i
n the resistance mechanism, suggesting that functions encoded by genes
unlinked to mar must be affected by marA. These results suggest that
activation of marA is the ultimate event that occurs at the mar locus
during the process that results in multiple antibiotic resistance.