Comparative genetics of capsular polysaccharide biosynthesis in Streptococcus pneumoniae types belonging to serogroup 19

The genetic basis for the structural diversity of capsule polysaccharide (C PS) in Streptococcus pneumoniae serogroup 19 (consisting of types 19F, 19A, 19B, and 19C) has been determined for the first time. In this study, the g enetic basis for the 19A and 19C serotypes is described, and the structures of all four serogroup 19 cps loci and their flanking sequences are compare d. Transformation studies show that the structural difference between the 1 9A and 19F CPSs is likely to be a consequence of differences between their respective polysaccharide polymerase genes (cps19aI and cps19fI). The CPS o f type 19C differs from that of type 19B by the addition of glucose. We hav e identified a single gene difference between the two cps loci (cp19cS), wh ich is likely to encode a glucosyl transferase. The arrangement of the gene s within the cps19 loci is highly conserved, with 13 genes (cps19A to -H an d cps19K to -O) common to all four serogroup 19 members. These cps genes en code functions required for the synthesis of the shared trisaccharide compo nent of the group 19 CPS repeat unit structures. Furthermore, the genetic d ifferences between the group 19 cps loci identified are consistent with the CPS structures of the individual serotypes. Functions have been assigned t o nearly all of the cps19 gene products, based on either gene complementati on or similarity to other proteins with known functions, and putative biosy nthetic pathways for production of all four group 19 CPSs have been propose d.