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Transport Systems for Alanine, Serine, and Glycine in Escherichia coli K-12

¹ Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48104

ABSTRACT

At least two transport systems serve for the entry of alanine, glycine, and serine into Escherichia coli. One of these systems serves mainly for glycine, D-alanine, and D-serine and to some extent for L-alanine, whereas the second serves for L-alanine and perhaps L-serine. These two transport systems have been characterized by kinetic studies and by inhibition analysis. Reciprocal plots for L-alanine entry are distinctly biphasic, giving rise to K_m values of about 2 and 27 µM. The major route of glycine entry can be described by a single K_m value of about 4 µM. A higher K_m value for glycine of around 70 to 100 µM shows that other routes of entry may serve at high concentrations of amino acid. The glycine, D-serine and D-alanine transport system is defective in a D-serine-resistant mutant, strain EM1302. The mutation, dagA, is recessive in dagA/dagA⁺ merodiploids and is 7 to 12% linked by phage P1 transduction to the pyrB locus of E. coli. E. coli with the dagA mutation are unable to utilize D-alanine as a carbon source, providing an additional basis for selecting such mutants. The remaining L-alanine uptake in dagA mutants is subject to inhibition by L-serine, L-threonine, and L-leucine. It is also sensitive to osmotic shock treatment and repressed by growth of the cells on L-leucine. It appears from a comparison of the properties of the second L-alanine system with those of the leucine, isoleucine, and valine system (LIV system) that the LIV system also serves for the transport of L-alanine and L-threonine and perhaps L-serine.

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