Requirement for phosphoglucomutase in exopolysaccharide biosynthesis in glucose- and lactose-utilizing Streptococcus thermophilus

To study the influence of phosphoglucomutase (PGM) activity on exopolysacch aride (EPS) synthesis in glucose- and lactose-growing Streptococcus thermop hilus, a knockout PGM mutant and a strain with elevated PGM activity were c onstructed. The pgmA gene, encoding PGM in S, thermophilus LY03, was identi fied and cloned. The gene was functional in Escherichia coli and was shown to be expressed from its own promoter. The pgmA-deficient mutant was unable to grow on glucose, while the mutation did not affect growth on lactose, O verexpression of pgmA had no significant effect on EPS production in glucos e-growing cells. Neither deletion nor overexpression of pgmA changed the gr owth or EPS production on lactose, Thus, the EPS precursors in lactose-util izing S. thermophilus are most probably formed from the galactose moiety of lactose via the Leloir pathway, which circumvents the need for a functiona l PGM.