ABSTRACT
Class I HAS belongs to the GT2 family of glycosyltransferases, which include other β-glycosyltransferases such as cellulose and chitin synthases (Griffiths et al., 1998), and it has been proposed that metazoan HAS evolved through the addition of β-1-3 glyco-syltransferase activity to a pre-existing β-1-4 glycosyltransferase enzyme (cellulose or chitin synthase) based on sequence homology and residual activity of some HAS enzymes (Lee and Spicer, 2000). The subgroup of the GT2 family that includes streptococcal and ver-tebrate HAS also includes rhizobial oligochitin synthases, a group of putative archaic glycyl transferases as well as some orphan genes (Blank et al., 2008). The finding of HAS genes on a Bacillus anthracis plasmid and in a viral genome indicates that HAS can be mobilized (Blank et al., 2008) and HAS was identified by the Human Genome Project as one of 223 candidates for lateral gene transfer from bac-teria to humans (Lander et al., 2001). Vertebrate HAS, however, is no more closely related to streptococcal HAS than to rhizobial oligochi-tin synthases or the putative archaic glycosyltranferases (Salzberg et al., 2001). Thus, streptococcal HAS may have evolved from cellu-lose or chitin synthase in a similar manner to metazoan HAS, but in a separate event. 16.3 Streptococcal Hyaluronic Acid ProductionHA for commercial applications is produced by extraction from animal tissues (e.g., rooster comb, bovine eyes, and umbilical cord) or through microbial fermentation. Concerns regarding the potential contamination of animal tissues with adventitious agents have seen the market shift toward microbial production.HA is produced as an exopolysaccharide coat by group A Streptococcus pyogenes and several group C streptococci, including Streptococcus equi, Streptococcus uberis, and Streptococcus equisimilis (Fig. 16.2). The HA capsule aids these (opportunistic) pathogens evade the host immune system and mutant microbes without HA capsules are less virulent than parental strains (Chung et al., 2001). The HA capsule may also aid migration of the bacterium through epi-thelial layers into tissue (Cywes and Wessels, 2001).The two HA precursors, UDP-GlcUA and UDP-GlcNAc, are synthesized from glucose-6-phosphate and fructose-6-phosphate, respectively (Fig. 16.3). The same pathways are used in the biosynthesis of cell wall constituents: cell wall polysaccharides,
teichoic acid, and peptidoglycan. In streptococci, the HAS is expressed from a polycistronic operon containing one or more of the enzymes responsible for precursor biosynthesis. Unlike Bacillus subtilis, streptococci do not produce teichuronic acid and the has operons
characterized to date all included hasB responsible for the final step in UDP-GlcUA biosynthesis. This minimal two-gene operon is found in S. uberis (Ward et al., 2001). The has operon in S. pyogenes includes an additional gene, hasC, obtained through duplication of galU (Dougherty and van de Rijn 1994). The operons in S. equi subspecies also contain hasC obtained through duplication of galU (Blank et al., 2008); sequence analysis indicates that this duplication event is distinct from that in S. pyogenes, but occurred prior to subspeciation into S. equi subsp. equi (S. equi) and S. equi subsp. zooepidemicus (S. zooepidemicus). The S. zooepidemicus operon includes a duplicated glmU gene (hasD) and is located immediately upstream of pgi, which is transcribed both from the has operon promoter and its own promoter (Blank et al., 2008).
