title: Sulfonamide resistance in Streptococcus pneumoniae: DNA sequence of the gene encoding dihydropteroate synthase and characterization of the enzyme.
abstract: 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.
mesh_terms: D001412:Bacillus subtilis / Q000235:genetics; D001483:Base Sequence; D055598:Chemical Phenomena; D002621:Chemistry; D003001:Cloning, Molecular; D004269:DNA, Bacterial / Q000032:analysis*; D004094:Dihydropteroate Synthase / Q000235:genetics* / Q000378:metabolism; D004352:Drug Resistance, Microbial / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics; D005798:Genes, Bacterial; D009154:Mutation; D013296:Streptococcus pneumoniae / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics; D013449:Sulfonamides / Q000494:pharmacology*; D014166:Transferases / Q000235:genetics*
chemical_list: D004269:DNA, Bacterial; D013449:Sulfonamides; D014166:Transferases; D004094:Dihydropteroate Synthase