Final Technical Report: Genetic Control of Nitrogen Assimilation in Klebsiella oxytoca.

Abstract

Klebsiella oxytoca, an enterobacterium closely related to Escherichia coli and amenable to molecular genetic analysis, is a long-established model organism for studies of bacterial nitrogen assimilation. Our work concerned utilization of purines, nitrogen-rich compounds that are widespread in the biosphere. This project began with our observation that molybdenum cofactor (chlorate-resistant) mutants can use (hypo)xanthine as sole nitrogen source (Garzon et al., J. Bacteriol. 174:6298, 1992). Since xanthine dehydrogenase is a molybdoenzyme, Klebsiella must use an alternate route for (hypo)xanthine catabolsim. We identified and characterized a cluster of 22 genes that encode the enzymes, permeases and regulators for utilizing hypoxanthine and xanthine as sole nitrogen source. (Hypoxanthine and xanthine arise from deamination of adenine and guanine, respectively.) Growth and complementation tests with insertion mutants, combined with protein sequence comparisons, allow us to assign probable functions for the products of these genes and to deduce the overall pathway. We present genetic evidence that the first two enzymes for the Klebsiella purine utilization pathway have been recruited from pathways involved in catabolism of aromatic compounds. The first, HxaAB enzyme catalyzing (hypo)xanthine oxidation, is related to well-studied aromatic ring hydroxylating oxygenases such as phthalate dioxygenase. The second, HxbA enzyme catalyzing urate hydroxylation, is relatedmore » to single-component monooxygenases. Thus, the Klebsiella purine utilization pathway has likely experienced non-orthologous gene displacement, substituting these oxygenases for the conventional enzymes, xanthine dehydrogenase and uricase. We also present evidence that transcription of the hxaAB operon is subject to dual regulation: global general nitrogen regulation (Ntr) through an unknown mechanism, and (hypo)xanthine induction mediated by a LysR-type activator.« less