Adaptation to sulfonamide resistance in Neisseria meningitidis may have required compensatory changes to retain enzyme function: kinetic analysis of dihydropteroate synthases from N. meningitidis expressed in a knockout mutant of Escherichia coli

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Abstract
Previously, the effects of three point mutations (at amino acid positions 31, 84, and 194) in the gene coding for a sulfonamide-resistant dihydropteroate synthase of Neisseria meningitidis were analyzed by site-directed mutagenesis. Changes at positions 31 and 194 abolished the phenotypic expression of sulfonamide resistance, while a change at position 84 appeared to be neutral. These studies are here extended to correlate the alterations in phenotype with effects on enzyme kinetics by expressing the cloned meningococcal genes in an Escherichia coli strain that had its dhps gene partially deleted and replaced by a resistance determinant. The most dramatic effects were produced by mutations at position 31. A change from the Leu found in the resistant isolate to a Phe (the residue found in sensitive isolates) led to a 10-fold decrease in the Km and a concomitant drop in the Ki. Changes at position 194 also affected both the Km and Ki but not to the same extent as mutations at position 31. Changing position 84 altered the Km only slightly but significantly. This latter change was interpreted as a compensatory change modulating the function of the enzyme. In another type of resistance gene, 2 amino acid residues, proposed to be an insertion, were deleted, resulting in a sensitive enzyme. However, the resulting Km was raised 10-fold, suggesting that compensatory changes have accumulated in this type of resistance determinant as well. This resistance gene differs by as much as 10% from the sensitive isolates, which makes identification of important mutations difficult.