Molecular characterization of hasB from an operon required for hyaluronic acid synthesis in group A streptococci. Demonstration of UDP-glucose dehydrogenase activity

The membrane-associated hyaluronate synthase produces capsular hyaluronate in group A streptococci by the alternate addition of UDP-N-acetylglucosamine and UDP-glucuronic acid. Previous studies identified a locus required for hyaluronate synthase activity and suggested that a gene involved in the production of UDP-glucuronic acid (UDP-glucose dehydrogenase) also mapped to the locus. In the present study the putative UDP-glucose dehydrogenase gene (hasB) was cloned and the DNA sequence determined. The hasB gene product was shown to have global similarity with AlgD, a dehydrogenase, which catalyzes the production of GDP-mannuronic acid for the alginate capsule of Pseudomonas aeruginosa. Regions of local homology have been identified which apparently correspond to the NAD-binding and enzyme active sites of HasB and AlgD. In order to show that hasB expression correlated with UDP-glucose dehydrogenase activity, the hasB gene was cloned under control of the T7 promoter. Hyperexpression of hasB resulted in a protein of approximately 47 kDa and high levels of UDP-glucose dehydrogenase activity were observed. These data demonstrate that hasB encodes the UDP-glucose dehydrogenase of group A streptococci.