Structure of CARB-4 and AER-1 CarbenicillinHydrolyzing β-Lactamases

We determined the nucleotide sequences ofblaCARB-4 encoding CARB-4 and deduced a polypeptide of 288 amino acids. The gene was characterized as a variant of group 2c carbenicillin-hydrolyzing β-lactamases such as PSE-4, PSE-1, and CARB-3. The level of DNA homology between thebla genes for these β-lactamases varied from 98.7 to 99.9%, while that between these genes andblaCARB-4 encoding CARB-4 was 86.3%. TheblaCARB-4 gene was acquired from some other source because it has a G+C content of 39.1%, compared to a G+C content of 67% for typicalPseudomonas aeruginosa genes. DNA sequencing revealed thatblaAER-1 shared 60.8% DNA identity withblaPSE-3 encoding PSE-3. The deduced AER-1 β-lactamase peptide was compared to class A, B, C, and D enzymes and had 57.6% identity with PSE-3, including an STHK tetrad at the active site. For CARB-4 and AER-1, conserved canonical amino acid boxes typical of class A β-lactamases were identified in a multiple alignment. Analysis of the DNA sequences flankingblaCARB-4 andblaAER-1 confirmed the importance of gene cassettes acquired via integrons inbla gene distribution.