The effect of substitution of asparagine for arginine at position 276 (Ambler's numbering) on the properties of the extended-spectrum β-lactamase CTX-M-4 was studied. Compared with CTX-M-4, the mutant β-lactamase CTX-M-4(R276N) conferred lower levels of resistance to cefotaxime, ceftriaxone and aztreonam while the levels of resistance to penicillins and penicillin-inhibitor combinations were similar. Arg-276→Asn substitution rendered CTX-M-4 slightly less susceptible to inhibition by clavulanate and tazobactam. It also caused a three-fold reduction in the relative rate of hydrolysis of cefotaxime. These results indicate that Arg-276 in CTX-M-type β-lactamases may be implicated in hydrolysis of oxyimino-β-lactams; they do not, however, support the hypothesis that Arg-276 is the functional equivalent of Arg-244 found in other class A β-lactamases. Copyright (C) 1998 Federation of European Microbiological Societies
