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%,

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... ompared to a G+C content of 67% for typical Pseudomonas aeruginosa genes. DNA sequencing revealed that blaAER-1 shared 60.8% DNA identity with blaPSE-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 and blaAER-1confirmed the importance of gene cassettes acquired via integrons inbla gene distribution.