Alignment of 12 class B β-lactamases numbered according to the BBL scheme. The sequences are referred to by their familiar names. BcII, Bacillus cereus 569H (15); IMP-1, Pseudomonas aeruginosa 101/477 (17); CcrA, Bacteroides fragilis TAL3636 (24); VIM-1, Pseudomonas aeruginosa VR-143/97 (18); BlaB, Chryseobacterium meningosepticum NCTC10585 (26); IND-1, Chryseobacterium indologenes 001 (3); CphA, Aeromonas hydrophila AE036 (20); Sfh-I, Serratia fonticola UTAD54 (GenBank accession no. AF197943); L1, Stenotrophomonas maltophilia IID1275 (32); FEZ-1, Legionella gormanii ATCC33297^T (5); GOB-1, Chryseobacterium meningosepticum PINT (2); and THIN-B, Janthinobacterium lividum JAC1 (25a). The names written in bold refer to the enzymes for which the three-dimensional structure is known. The amino acid in bold (Ala 22 of L1) represents the first amino acid of the mature β-lactamase. Conserved secondary structure elements of subclasses B1 and B3 are indicated above the sequences: 3₁₀, 3₁₀ helix; S, β strand; H, helix. Secondary structure elements specific to subclasses B1 and B3 are highlighted by italic characters above and under the sequences, respectively. Amino acid insertions in newly sequenced enzymes are represented by small letters. The residues acting as zinc ligands in at least one subclass are characterized as follows: z, conserved residues in the three subclasses; ·, conserved residues in subclass B1 and some enzymes of subclass B3; +, conserved residue in subclass B3; §, conserved residues in subclasses B1 and B2.