Xanthomonas campestris pv. campestris, the causal agent of black-rot disease of cruciferous plants, and an important industrial microbe, was able to express the Escherichia coli beta-glucuronidase reporter gene (uidA) when fused to the E. coli lactose operon promoter on a wide-host-range plasmid vector. The gene fusion is expressed constitutively at high levels in both complex and defined media using a wide range of carbon sources, and is not repressible by glucose or inducible by the gratuitous lac inducer isopropyl beta-D-thiogalactoside. An X. camprestris campestris strain with a lesion in the clp (catabolite-repressor-like protein) locus, and containing the plac/uidA fusion, was tested for beta-glucuronidase activity. We found that the expression of the plac/uidA fusion gene is dependent on the presence of catabolite-repressor-like protein, with an approximately 75% reduction of expression in the clp -deficient mutant.