Common mechanism of ampC β-lactamase induction in enterobacteria: Regulation of the cloned Enterobacter cloacae P99 β-lactamase gene

Abstract

Expression of the chromosomal β-lactamase from the ampC gene is inducible in both Enterobacter cloacae and Citrobacter freundii. Cloning of ampC as well as its regulatory gene, ampR, from E. cloacae P99 revealed a gene organization indentical to that of C. freundii in the corresponding region. Although almost no similarities could be found between the restriction maps of ampC and ampR in the two species, the genes cross-hybridize. Also, both ampR gene products have a size of about 31,000. The regulatory features of E. cloacae β-lactamase induction are very similar to those in C. freundii, i.e., β-lactamase synthesis is repressed by AmpR in the absence, and stimulated in the presence, of inducer. The AmpR function can be transcomplemented between the two species, but there are quantitative regulatory aberrations in such hybrids, in contrast to the total complementation obtained within each system. These results suggest that the mechanism of β-lactamase induction is the same in E. cloacae, C. freundii, and other gram-negative bacteria with inducible chromosomal β-lactamase expression.