Development of gene probes and evolutionary relationships of the PSE-4 bla gene to plasmid-mediated beta-lactamases of gram-negative bacteria.

Six types of plasmid-mediated carbenicillinases can be distinguished on the basis of their substrate profiles, molecular mass isoelectric values and immunological properties. As yet, no structural classification has been attempted for these enzymes at the molecular level. We have isolated the PSE-4 structural gene responsible for carbenicillinase production in Pseudomonas aeruginosa strain Dalgleish and studied its expression in E. coli. A detailed physical map of the cloned fragment and the construction of deletion mutants permitted the precise localization of the PSE-4 structural gene. Various restriction endonuclease fragments known to be flanking or internal to the PSE-4 bla gene were used as DNA probes and tested for homologous sequences in other beta-lactamase genes. A collection of three restriction fragment probes internal or delimiting the PSE-4 structural gene were hybridized with purified plasmid DNA coding for 18 other beta-lactamases. Under high stringency conditions, only the PSE-1, CARB-3 and CARB-4 genes cross-hybridized with PSE-4; while one of the probes tested hybridized solely with CARB-3. Further analysis indicated that the PSE-1, PSE-4, CARB-3 and CARB-4 bla genes are related and could presumably have evolved from a common progenitor.