Experiments were conducted to determine the pathway of melamine metabolism by Klebsiella terragena (strain DRS-1) and the effect of added NH(inf4)(sup+) on the rates and extent of melamine metabolism. In the absence of added NH(inf4)(sup+), 1 mM melamine was metabolized concomitantly with growth. Ammeline, ammelide, cyanuric acid, and NH(inf4)(sup+) accumulated transiently in the culture medium to maximal concentrations of 0.012 mM, 0.39 mM, trace levels, and 0.61 mM, respectively. In separate incubations, in which cells were grown on either ammeline or ammelide (in the absence of NH(inf4)(sup+)), ammeline was metabolized without a lag while ammelide metabolism was observed only after 3 h. In the presence of 6 mM added NH(inf4)(sup+) (enriched with 5% (sup15)N), ammeline, ammelide, and cyanuric acid accumulated transiently to maximal concentrations of 0.002 mM, 0.47 mM, and trace levels, respectively, indicating that the added NH(inf4)(sup+) had little effect on the relative rates of triazine metabolism. These data suggest that the primary mode of melamine metabolism by K. terragena is hydrolytic, resulting in successive deaminations of the triazine ring. Use of (sup15)N-enriched NH(inf4)(sup+) allowed estimates of rates of triazine-N mineralization and assimilation of NH(inf4)(sup+)-N versus triazine-N into biomass. A decrease in the percent (sup15)N in the external NH(inf4)(sup+) pool, in conjunction with the accumulation of ammelide and/or triazine-derived NH(inf4)(sup+) in the culture medium, suggests that the initial reactions in the melamine metabolic pathway may occur outside the cytoplasmic membrane
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