title: Roles of individual mgl gene products in the beta-methylgalactoside transport system of Escherichia coli K12.
abstract: Previous findings showed that galactose-binding protein defective mutants (mgl B-,A+,C+) of Escherichia coli K12 are still capable of growth on methyl-beta-D-galactopyranoside, while mgl A- and mgl C- mutants are not. When assayed by previous methods, none of these mutants exhibited methylgalactoside transport system activity. In this study, we present a modified assay developed for measuring low levels of transport. Using this assay, we found that mgl B-,A+,C+ mutants defective in galactose-binding protein accumulate methyl-beta-D-galactopyranoside up to six times the concentration gradient while mgl A- and mgl C- mutants failed to accumulate this substrate. Similar results were obtained using D-glyceryl-beta-D-galactopyranoside, another substrate of the methylgalactoside transport system. In contrast, all sugars tested which are not substrates of this system were transported equally by all mgl- mutants. The kinetic parameters of transport in mgl B- mutants were compared to those of the isogenic mgl+ strain which accumulates methyl-beta-D-galactopyranoside against a 10,000-fold concentration gradient. The apparent Km of methyl-beta-D-galactopyranoside influx was 1,000 times greater in mgl B- than in mgl+ strains. In contrast, there was no significant difference between these strains in either the Vmax of substrate influx or the rate of substrate exit. D-Galactose competitively inhibited methyl-beta-D-galactopyranoside influx into both mgl B- and mgl+ strains; the Ki of inhibition in mgl B- cells was 2,000-fold greater than that in mgl+ cells.
mesh_terms: D001693:Biological Transport, Active; D002352:Carrier Proteins / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism*; D005690:Galactose / Q000494:pharmacology; D005697:Galactosides / Q000378:metabolism; D005796:Genes; D005989:Glycerides / Q000378:metabolism; D007700:Kinetics; D026901:Membrane Transport Proteins / Q000378:metabolism; D008756:Methylgalactosides / Q000378:metabolism*; D008759:Methylglycosides / Q000378:metabolism*; D009154:Mutation
chemical_list: D002352:Carrier Proteins; D005697:Galactosides; D005989:Glycerides; D026901:Membrane Transport Proteins; D008756:Methylgalactosides; D008759:Methylglycosides; D005690:Galactose