Characterization of a novel SHV beta-lactamase variant that resembles the SHV-5 enzyme.

An SHV type beta-lactamase frequently found in enterobacteria isolated in Greek hospitals was analyzed. The enzyme (SHV-5a) conferred resistance to ceftazidime and aztreonam. The DNA sequence of the structural gene was determined. The deduced amino acid sequence showed that positions 70-73 were occupied by the active site tetrad Ser-Thr-Phe-Lys. As in SHV-5, Ser-238 and Lys-240 were present. However, one deletion (Gly-54) and three substitutions (Arg-140 for Ala, Asn-192 for Lys and Val-193 for Leu) differentiate SHV-5a beta-lactamase from SHV-5. Asn-192 and Val-193 have been reported to date only in the R974 plasmid-mediate SHV-1 beta-lactamase. Hydrolysis studies with SHV-5a and SHV-5 showed that the enzymes behaved similarly. Additional evidence that they are functionally indistinguishable was provided by the similar MICs of beta-lactams when the enzymes were expressed under isogenic conditions. The sequence differences, however, indicate that they are derived from different ancestors.