title: Transport of sugars via two anomer-specific sites on mannose-phosphotransferase system in Lactococcus cremoris: in vivo study of mechanism, kinetics, and adaptation.
abstract: Glucose uptake in Lactococcus lactis subsp. cremoris FD1 occurs via the mannose phosphotransferase system (Man-PTS), which is quite unspecific and allows transport of many different sugars and sugar analogues. It was previously shown (Benthin, S., Nielsen, J., Villadsen, J. Biotechnol. Bioeng. 40:137-146, 1992) that the kinetics of in vivo glucose uptake in a glucose-limited chemostat culture is best described by assuming that the glucose transport system has two anomer-specific sites with a relative uptake rate of 36% through the alpha-site. In the present study, the existence of anomer-specific sites on Man-PTS is shown by experiments where alpha-glucose, beta-glucose, mannose, and 2-deoxyglucose are added to glucose-limited chemostat cultures. A quantitative description of the competitive uptake of the involved sugars at the two sites is given. In a mannose-limited chemostat culture, the relative glucose flux via the alpha-site is 50%, corresponding to a change toward the equilibrium composition of mannose (68%). Furthermore, when the feed to a mannose-limited chemostat culture is changed to glucose, the rate of change of relative glucose flux through the alpha-site corresponds to constitutive synthesis of Man-PTS with 36% alpha-site stoichiometry in new cells. When N-acetylglucosamine (73% alpha-anomer at equilibrium) is the limiting substrate, the relative glucose flux through the alpha-site is also 48% to 50%. With a feed of alpha-glucose generated enzymatically from nonmetabolizable sucrose the relative glucose flux through the alpha-site can be as high as 78%. Finally, growth in the presence of nonmetabolizable alpha-methylglucoside leads to formation of cells with a relative glucose flux through the alpha-site of 29% to 30%. The adaptation of the flux distribution between the alpha- and beta-site is tentatively explained by the hypothesis that two integral membrane proteins of Man-PTS are involved in this process.
authors.affiliation: Department of Biotechnology, The Technical University of Denmark, Lyngby, Denmark.
authors.forename: S
authors.initials: S
authors.lastname: Benthin
authors.forename: J
authors.initials: J
authors.lastname: Nielsen
authors.forename: J
authors.initials: J
authors.lastname: Villadsen