Xanthomonas campestris pv. campestris (Xcc) is the causal agent of black rot disease of cruciferous plant. Previous studies demonstrated that the bla gene residing in the chromosome of Xcc encoding a beta-lactamase which hydrolyzes beta-lactam type antibiotics and renders Xcc antibiotic resistant. The ampR gene, adjacent to bla and oriented in the opposite direction, is essential for the expression of bla. To
elucidate the mechanism of the regulation of the antibiotic esistance in Xcc, mutants of bla (Xc17bla::Gm) and ampR (Xc17ampR::Gm) were constructed by insertional mutagenesis. The antibiotic resistance was assayed by measuring the minimal inhibition concentration (MIC) in the presence of different antibiotics and the beta-lactamase activity. The promoter regions of bla and ampR were constructed into
a promoter proving vector pFY13-9 to from a transcriptional fusion with a promoterless lacZ gene. The promoter activity was monitored by assaying the beta-galactosidase activity. Our data demonstrate that 1) The bla and ampR genes are essential for the beta-lactam type antibiotics resistance; 2) The expression of bla gene is inducible by beta-lactam type antibiotics and positively regulated by ampR in the transcriptional level; 3) Bla negatively regulates the expression of bla gene; 4) The LysR motif located upstream of bla promoter is essential for the induction of bla promoter by AmpR; 5) The ampR promoter expresses constitutively in the presence or absence of activator; 6) Expression of ampR gene is negatively regulated by AmpR; 7) LysR motif is essential for the expression of ampR gene.
