A binding protein-dependent transport system in Streptococcus mutans responsible for multiple sugar metabolism.

An 11-kilobase gene region of Streptococcus mutans has been identified which contains eight contiguous genes involved with the uptake and metabolism of multiple sugars (the msm system). Sequence analysis of this region indicates that several of these genes specify proteins with strong homology to components of periplasmic binding protein-dependent transport systems of Gram-negative bacteria. Additionally, this operon is controlled by a regulatory gene (msmR) that acts as a positive effector. The proteins specified by the structural genes of the msm operon include alpha-galactosidase (aga), a “periplasmic-like” sugar-binding protein (msmE), two membrane proteins (msmF, msmG), sucrose phosphorylase (gtfA), an ATP-binding protein (msmK), and dextran glucosidase (dexB). Insertional inactivation of each of these genes along with uptake data indicate that this system is responsible for the uptake of melibiose, raffinose, and isomaltotriose and the metabolism of melibiose, sucrose, and isomaltosaccharides.