Functional analysis in type Ia group B Streptococcus of a cluster of genesinvolved in extracellular polysaccharide production by diverse species of streptococci

Several species of streptococci produce extracellular polysaccharides in th e form of secreted exopolysaccharides or cell-associated capsules. Although the biological properties and repeating unit structures of these polysacch arides are diverse, sequence analysis of the genes required for their produ ction has revealed a surprising degree of conservation among five genes fou nd in the capsule gene cluster of each of several polysaccharide-producing streptococci. To determine the function of these conserved genes, we charac terized a series of isogenic mutants derived from a wild-type strain of typ e Is group B Streptococcus by selectively inactivating each gene. Inactivat ion of cpsIaE resulted in an aeapsular phenotype, consistent with previous work that identified the cpsIaE product as the glycosyltransferase that ini tiates synthesis of the polysaccharide repeating unit. Mutants in cpsIaA, c psIaB, cpsIaC, or cpsIaD produced type Is capsular polysaccharide, but in r educed amounts compared with the wild type. Analysis of the mutant polysacc harides and of capsule gene transcription in the mutant strains provided ev idence that cpsIaA encodes a transcriptional activator that regulates expre ssion of the capsule gene operon. Mutants in cpsIaC or cpsIaD produced poly saccharide of reduced molecular size but with an identical repeating unit s tructure as the wild-type strain. We conclude that CpsA to -D are not requi red for polysaccharide repeating unit biosynthesis but rather that they dir ect the coordinated polymerization and export of high molecular weight poly saccharide.