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J. Bacteriol. doi:10.1128/JB.01940-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Escherichia coli abg Genes Enable Uptake and Cleavage of Folate Catabolite p-Aminobenzoyl-glutamate

Department of Biochemistry, Chicago College of Osteopathic Medicine, Midwestern University, 555 31^st Street, Downers Grove, IL, 60515

^* To whom correspondence should be addressed. Email: jgreen{at}midwestern.edu.

	Abstract

Escherichia coli AbgT was first identified as a structural gene enabling growth of p-aminobenzoate auxotrophs to grow on exogenous p-aminobenzoyl-glutamate [Hussein, M. J., J. M. Green, and B. P. Nichols. 1998. J Bacteriol 180:6260-8]. The abg region includes abgA, abgB, abgT, and ogt; these genes may be regulated by AbgR, a divergently-transcribed LysR-type protein. Wild type cells transformed with a high copy plasmid encoding abgT demonstrate saturable uptake of p-aminobenzoyl-glutamate (Kt = 123 µM); control cells expressing vector demonstrate negligible uptake. Addition of metabolic poisons inhibited uptake of p-aminobenzoyl-glutamate, consistent with this process requiring energy. p-Aminobenzoyl-glutamate taken in by cells expressing high amounts of AbgT alone is not rapidly metabolized to a form that is trapped in the cell, as addition of non-radioactive p-aminobenzoyl-glutamate to these cells results in rapid loss of intracellular label. Addition of non-radioactive p-aminobenzoate has no effect. The abgA, abgB, and abgAB genes were cloned into medium copy pACYC184; p-aminobenzoate auxotrophs transformed with the clone encoding abgAB demonstrated enhanced ability to grow on low levels of p-aminobenzoyl-glutamate. When transformed with complementary plasmids encoding high copy levels of abgT and medium copy levels of abgAB, p-aminobenzoate auxotrophs grew on 50 nM p-aminobenzoyl-glutamate. Our data are consistent with a model of p-aminobenzoyl-glutamate utilization in which AbgT catalyzes transport of p-aminobenzoyl-glutamate, followed by cleavage to p-aminobenzoate by a protein composed of subunits encoded by abgA and abgB. While endogenous expression of these genes is very low under the conditions in which we have performed our experiments, these genes may be induced by AbgR bound to an unknown molecule. The true physiological role of this region may be related to some molecule similar to p-aminobenzoyl-glutamate, such as a dipeptide.