Amino Acid Repetitions in the Dihydropteroate Synthase of Streptococcus pneumoniae Lead to Sulfonamide Resistance with Limited Effects on Substrate K m

• Published in: Antimicrobial agents and chemotherapy Vol. 45; no. 3; pp. 805 - 809
• Main Authors: Haasum, Ylva, Ström, Katrin, Wehelie, Rahma, Luna, Vicki, Roberts, Marilyn C., Maskell, Jeffrey P., Hall, Lucinda M. C., Swedberg, Göte
• Format: Journal Article
• Language: English
• Published: 01-03-2001
• ISSN: 0066-4804; 1098-6596
• DOI: 10.1128/AAC.45.3.805-809.2001

ABSTRACT
Sulfonamide resistance in
Streptococcus pneumoniae
is due to changes in the chromosomal
folP
(
sulA
) gene coding for dihydropteroate synthase (DHPS). The first reported laboratory-selected sulfonamide-resistant
S. pneumoniae
isolate had a 6-bp repetition, the
sul-d
mutation, leading to a repetition of the amino acids Ile
66
and Glu
67
in the gene product DHPS. More recently, clinical isolates showing this and other repetitions have been reported. WA-5, a clinical isolate from Washington State, contains a 6-bp repetition in the
folP
gene, identical to the
sul-d
mutation. The repetition was deleted by site-directed mutagenesis. Enzyme kinetic measurements showed that the deletion was associated with a 35-fold difference in
K
i
for sulfathiazole but changed the
K
m
for
p
-aminobenzoic acid only 2.5-fold and did not significantly change the
K
m
for 2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine pyrophosphate. The enzyme characteristics of the deletion variant were identical to those of DHPS from a sulfonamide-susceptible strain. DHPS from clinical isolates with repetitions of Ser
61
had very similar enzyme characteristics to the DHPS from WA-5. The results confirm that the repetitions are sufficient for development of a resistant enzyme and suggest that the fitness cost to the organism of developing resistance may be very low.