Growth substrates containing an s-triazine ring are typically metabolized by bacteria to liberate 3 mol of ammonia via the intermediate cyanuric acid. Over a 25-year period, a number of original research papers and reviews have stated that cyanuric acid is metabolized in two steps to the 2-nitrogen intermediate urea. In the present study, allophanate, not urea, was shown to be the 2-nitrogen intermediate in cyanuric acid metabolism in all the bacteria examined. Six different experimental results supported this conclusion: (i) synthetic allophanate was shown to readily decarboxylate to form urea under acidic extraction and chromatography conditions used in previous studies; (ii) alkaline extraction methods were used to stabilize and detect allophanate in bacteria actively metabolizing cyanuric acid; (iii) the kinetic course of allophanate formation and disappearance was consistent with its being an intermediate in cyanuric acid metabolism, and no urea was observed in those experiments; (iv) protein extracts from cells grown on cyanuric acid contained allophanate hydrolase activity; (v) genes encoding the enzymes AtzE and AtzF, which produce and hydrolyze allophanate, respectively, were found in several cyanuric acid-metabolizing bacteria; and (vi) TrzF, an AtzF homolog found in Enterobacter cloacae strain 99, was cloned, expressed in Escherichia coli, and shown to have allophanate hydrolase activity. In addition, we have observed that there are a large number of genes homologous to atzF and trzF distributed in phylogenetically distinct bacteria. In total, the data indicate that s-triazine metabolism in a broad class of bacteria proceeds through allophanate via allophanate hydrolase, rather than through urea using urease.
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