A conserved UDP-glucose dehydrogenase encoded outside the hasabc operon contributes to capsule biogenesis in group A Streptococcus

Abstract

Group A Streptococcus (GAS) is a human-specific bacterial pathogen responsible for serious morbidity and mortality worldwide. The hyaluronic acid (HA) capsule of GAS is a major virulence factor, contributing to bloodstream survival through resistance toneutrophil and antimicrobial peptide killing and to in vivo pathogenicity. Capsule biosynthesis has been exclusively attributedto the ubiquitous hasABC hyaluronan synthase operon, which is highly conserved across GAS serotypes. Previous reports indicatethat hasA, encoding hyaluronan synthase, and hasB, encoding UDP-glucose 6-dehydrogenase, are essential for capsule productionin GAS. Here, we report that precise allelic exchange mutagenesis of hasB in GAS strain 5448, a representative of theglobally disseminated M1T1 serotype, did not abolish HA capsule synthesis. In silico whole-genome screening identified a putativeHasB paralog, designated HasB2, with 45% amino acid identity to HasB at a distant location in the GAS chromosome. Invitro enzymatic assays demonstrated that recombinant HasB2 is a functional UDP-glucose 6-dehydrogenase enzyme. Mutagenesisof hasB2 alone slightly decreased capsule abundance; however, a ΔhasB ΔhasB2 double mutant became completely acapsular. We conclude that HasB is not essential for M1T1 GAS capsule biogenesis due to the presence of a newly identified HasB paralog,HasB2, which most likely resulted from gene duplication. The identification of redundant UDP-glucose 6-dehydrogenases underscoresthe importance of HA capsule expression for M1T1 GAS pathogenicity and survival in the human host. © 2012, American Society for Microbiology.