Substitution of serine for arginine in position 162 of TEM-type beta-lactamases extends the substrate profile of mutant enzymes, TEM-7 and TEM-101, to ceftazidime and aztreonam.

TEM-7 is a novel broad-spectrum beta-lactamase (Bla), selected in vivo, with a resistance profile similar to that of TEM-1 and TEM-2, but extended to ceftazidime (Caz) and aztreonam. Nucleotide sequencing revealed that the TEM-7 gene is almost identical with that of TEM-2. There was 1 bp change which would result in the substitution of Ser (TEM-7) for Arg (TEM-2) in amino acid (aa) position 162 (i.e., aa position 139 of the mature enzyme). This substitution, also found in TEM-101, a spontaneous in vitro derivative of TEM-1 selected on Caz, was assumed to be responsible for the extension of the substrate profile. The assumption was verified by exchange of a DNA fragment, carrying the mutation of the TEM-7-coding gene, with the homologous fragment of the TEM-1-coding gene in pBR322. In the three-dimensional model of class-A Bla [Joris et al., Biochem. J. 250 (1988) 313-324], aa 139 is located at the rim of the groove which contains the active center and adjacent to the evolutionarily conserved BoxV. It is speculated that extra free hydroxyl groups in this area may participate in the stabilization of otherwise non-substrate compounds.