CHARACTERIZATION OF A MUTATIONALLY ALTERED DIHYDROPTEROATE SYNTHASE CONTRIBUTING TO SULFATHIAZOLE RESISTANCE IN ESCHERICHIA-COLI

A series of Escherichia coli strains were selected for increasing resi stance to sulfathiazole. Resistance occurred in seven increments, sugg esting the accumulation of several mutations that contributed to overa ll sulfathiazole resistance, All of the resistant strains had a sulfat hiazote-resistant dihydropteroate synthase with a Pro to Ser substitut ion at amino acid position 64, Overproduction of the wild-type enzyme did not result in sulfathiazole resistance, however overproduction of the mutant enzyme resulted in significant resistance, Conversely, over production of the wild-type enzyme in a sulfathiazole-resistant backgr ound resulted in a decrease in resistance, Although the specific activ ity of DHPS in crude extracts was not significantly different from the wild type, the amino acid substitution resulted in an enzyme with a t enfold increase in the Km for p-aminobenzoate, and a 100-fold increase in the Ri for sulfathiazole.