Xanthomonas campestris pv. campestris ( Xcc ) is the causal agent of black rot disease of cruciferous plant. Previous studies demonstrated that the chromosomally encoded Bla β-lactamase of Xcc can hydrolyze β-lactam antibiotics, resulting in a resistance to β-lactam antibiotics. Transcription of bla is dependent on the transcription regulator AmpR divergently transcribed from the ampR gene resided upstream of bla. In Gram negative bacteria, activation of inducible β-lactamase is closely related to the cell wall (peptidoglycan) recycling. The turnover products of peptidoglycan, such as 1,6-GlcNAc-anhydro-MurNAc, and 1,6-GlcNAc-anhydro-MurNAc peptide, are transported from the periplasmic space to cytosol by AmpG permease. The nagZ gene encoding a β-N-acetylglucosaminidase removes the GlcNAc molecule from GlcNAc-anhMurNAc-tripeptide to produce anhMurNAc-tripeptide, which can be further processed by several enzymes to form the precursor of peptidoglycan. AnhMurNAc-tripeptide can also bind to AmpR to activate the expression of β-lactamase. To study the roles of AmpG and NagZ on the expression of bla expression in Xcc, mutants of ampG and nagZ were constructed by insertional mutagenesis. The ampG and nagZ mutants become sensitive to ?-lactam antibiotics and fail to express bla gene and produce??-lactamase, suggesting that the peptidoglycan turnover product is required for the activation of bla in Xcc.
