Induction of a secretable beta-lactamase requires a long lag time in Xanthomonas campestris pv. campestris str. 17.

Xanthomonas campestris pv. campestris (Xcc) constitutively expresses penicillinase activity in the absence of an inducer. An ampR-bla module is required for the antibiotic resistance phenotype. In this study, we demonstrate that AmpR negatively autoregulates its own expression in a β-lactam-independent manner. In the absence of inducer, bla is expressed at a high basal level. Expression of bla is inducible by β-lactam, however, with a period. AmpR protein and the LysR-motif located upstream of bla promoter are essential for basal expression and induction of bla. Most β-lactamase activity is present in the culture medium, suggesting that Bla protein can be secreted by Xcc into the environment.