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

We determined the nucleotide sequences of bla(CARB-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 the bla genes for these β-lactamases varied from 98.7 to 99.9%, while that between these genes and bla(CARB-4) encoding CARB-4 was 86.3%. The bla(CARB-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 typical Pseudomonas aeruginosa genes. DNA sequencing revealed that bla(AER-1) shared 60.8% DNA identity with bla(PSE-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 flanking bla(CARB-4) and bla(AER-1) confirmed the importance of gene cassettes acquired via integrons in bla gene distribution.