Capsule biosynthesis and basic metabolism in Streptococcus pneumoniae are linked through the cellular phosphoglucomutase

Synthesis of the type 3 capsular polysaccharide of Streptococcus pneumoniae requires UDP-glucose (UDP-Glc) and UDP-glucuronic acid (UDP-GlcUA) for pro duction of the [3)-beta-D-GlcUA-(1-->4)-beta-D-Glc-(1-->](n) polymer, The g eneration of UDP-Glc proceeds by conversion of Glc-6-P to Glc-1-P to UDP-Gl c and is mediated by a phosphoglucomutase (PGM) and a Glc-1-P uridylyltrans ferase, respectively. Genes encoding both a Glc-1-P uridylyltransferase (cp s3U) and a PGM homologue (cpsM) are present in the type 3 capsule locus, bu t these genes are not essential for capsule production. In this study, we c haracterized a mutant that produces fourfold less capsule than the type 3 p arent. The spontaneous mutation resulting in this phenotype was not contain ed in the type 3 capsule locus but was instead located in a distant gene (p gm) encoding a second PGM homologue. The function of this gene product as a PGM was demonstrated through enzymatic and complementation studies. Insert ional inactivation of pgm reduced capsule production to less than 10% of th e parental level. The loss of PGM activity in the insertion mutants also ca used growth defects and a strong selection for isolates containing second-s ite suppressor mutations. These results demonstrate that most of the PGM ac tivity required for type 3 capsule biosynthesis is derived from the cellula r PGM.