Early studies involving purine salvage in Salmonella typhimurium resulted in the isolation and identification of a mutant strain possessing a genetically modified hypoxanthine phosphoribosyl-transferase (HPRT) with enhanced substrate specificity for guanine [Benson, C. E., and Gots, J. S. (1975) J. Bacteriol. 121, 77-82]. To explore the molecular basis for this altered substrate specificity in the mutant hpt gene product, degenerate oligonucleotide primers, designed according to the N- and C-termini of the HPRT of Escherichia coli, were used in polymerase chain reactions to amplify both the mutant and wild-type S. typhimurium hpt genes from genomic DNA. Analysis of the deduced amino acid sequences revealed that a single base mutation resulted in the encoding of a Thr in the mutant HPRT, instead of an Ile found in the wild-type enzyme, at a position analogous to position 192 (Leu-192) of the human HPRT. Comparison of kinetic data for purified recombinant mutant and wild-type HPRTs showed no difference in the overall catalytic efficiency (kcat/K(m)) with hypoxanthine as substrate, but with guanine, the mutant enzyme exhibited a more than 50-fold higher kcat/K(m) largely as a result of a decrease of nearly 2 orders of magnitude in K(m). Involvement in substrate binding of the cognate amino acid at position 192 in the human HPRT was investigated using site-directed mutagenesis. Mutation of Leu-192 to Thr did not significantly alter kcat/K(m) values for hypoxanthine and guanine compared to wild-type, and replacement of Leu-192 with Ile had no significant change in kinetics for either hypoxanthine or PRPP. However, this Ile substitution resulted in an over 15-fold decrease in the kcat/K(m) for guanine due to a greater than 15-fold increase in K(m). These results demonstrate that a single active site amino acid substitution in HPRTs can significantly alter the specificity for binding guanine.