Evidence for binding protein-independent substrate translocation by the methylgalactoside transport system of Escherichia coli K12.

Three genes, mgl A, B, and C, are required for active transport of substrate by the methylgalactose permease of E. coli K12. We report here that only two of these genes are required for substrate translocation, as seen by the ability or inability of isogenic mgl mutants (referred to as Tra+ and Tra minus, respectively) to grow on methyl-beta-D-galactopyranoside, supplied as sole carbon source. Individual mutants of both the Tra+ and Tra minus classes exhibited no detectable intracellular accumulation of methyl-beta-D-galactopyranoside; thus, the Tra+ phenotype cannot be explained by the mutants' levels of residual active transport. The phosphotransferase (Pts), the beta-galactoside (LacY), and the arabinose (Ara E and Ara F) transport systems are not required for substrate translocation by Tra+ cells. The Tra+ phenotype was identified with mutants defective in the mgl B, locus of the galactose-binding protein, by genetic complementation; the Tra minus phenotype was observed with both mgl A and mgl C mutants. The conclusion that the galactose-binding protein is not required for substrate translocation was supported by direct assays of the mgl mutants' binding protein activity. Mutants capable of translocation all showed reduced galactose-binding protein activity; mutants incapable of translocation exhibited binding protein activity equal to that of the mgl+ parent.