Sequences of Homologous β-Lactamases from Clinical Isolates of Serratia marcescens with Different Substrate Specificities

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Abstract
Genes for two group 1 β-lactamases, SRT-1 and SST-1, were sequenced. These β-lactamases were produced by clinical isolates of Serratia marcescens , isolates GN16694 and GN19450, respectively. The resulting enzymes were 96% identical. SRT-1 hydrolyzed oxyimino cephalosporins, but SST-1 hardly hydrolyzed them. At residue 213 in the third motif, which is conserved among group 1 β-lactamases, SRT-1 and SST-1 had Lys and Glu, respectively. By site-directed mutagenesis, the substitution of Glu by Lys at residue 213 in SST-1 resulted in an enzyme that hydrolyzed oxyimino cephalosporins.