Promoter sequences necessary for high-level expression of the plasmid-associated ampC β-lactamase gene blaMIR-1

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Little is known about mechanisms involved in high-level expression of plasmid-associated ampC genes. The sequence for blaMIR-1 has been elucidated, and the gene is not inducible. Although the sequence for the promoter (prA) that drives expression of Enterobacter cloacae chromosomal ampC is present upstream of blaMIR-1, high-level expression from bla MIR-1 is directed from a hybrid promoter (prB) located further upstream of prA. The purpose of this study was to determine the influence of each promoter on blaMIR-1 expression and β-lactam resistance. RNA expression by deletion clones with both promoters was measured and compared to that by clones in which -35 and/or -10 elements of prA and/or prB were altered. Primer extension revealed two start sites for blaMIR-1 transcription. Expression of blaMIR-1 in clones with both promoters was 171-fold higher than that in clones carrying only prA. In addition, bla MIR-1 expression from prA increased 11-fold in the presence of the prB -10 element compared to expression driven from prA alone. Ceftazidime and cefotaxime MICs increased 42- and 64-fold, respectively, for the clone expressing blaMIR-1 from both promoters compared to expression from prA alone. The upstream promoter prB of blaMIR-1 is solely responsible for high-level expression required for cefotaxime and ceftazidime resistance. These data suggest that resistance to extended-spectrum cephalosporins mediated by noninducible plasmid-associated ampC genes requires the formation of novel promoter elements that are capable of increasing ampC expression.

Original languageEnglish (US)
Pages (from-to)4177-4182
Number of pages6
JournalAntimicrobial agents and chemotherapy
Issue number11
StatePublished - Nov 2004

All Science Journal Classification (ASJC) codes

  • Pharmacology
  • Pharmacology (medical)
  • Infectious Diseases


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