Sulfonamide resistance in Streptococcus pneumoniae: DNA sequence of the gene encoding dihydropteroate synthase and characterization of the enzyme.

A chromosomal gene of Streptococcus pneumoniae carrying a spontaneous mutation to sulfonamide resistance was identified. Comparison of its DNA sequence with the wild-type sequence showed that the mutation, sul-d, consisted of an insert of 6 base pairs, a repeat of an adjacent 6-base-pair segment. The gene encoded a 34-kilodalton polypeptide, SulA, which as a dimer or trimer constituted the enzyme dihydropteroate synthase. This was shown by enzyme activity measurements, expression in minicells of Bacillus subtilis, and the amino-terminal sequence of the polypeptide product. Subcloning of the gene in an Escherichia coli expression vector allowed purification of the enzyme to 80% homogeneity in a single step and at high yield. Although a deleted plasmid, pLS83, produced the mutant dihydropteroate synthase, it did not confer sulfonamide resistance in vivo. It is suggested that the SulA polypeptide is also a component of an enzyme that acts in another step of folate biosynthesis and that this step is inhibited in vivo by either free or conjugated sulfonamides.