Novel Route for Agmatine Catabolism in Aspergillus niger Involves 4-Guanidinobutyrase
ABSTRACTAgmatine, a significant polyamine in bacteria and plants, mostly arises from the decarboxylation of arginine. The functional importance of agmatine in fungi is poorly understood. The metabolism of agmatine and related guanidinium group-containing compounds inAspergillus nigerwas explored through growth, metabolite, and enzyme studies. The fungus was able to metabolize and grow onl-arginine, agmatine, or 4-guanidinobutyrate as the sole nitrogen source. Whereas arginase defined the only route for arginine catabolism, biochemical and bioinformatics approaches suggested the absence of arginine decarboxylase inA. niger. Efficient utilization by the parent strain and also by its arginase knockout implied an arginase-independent catabolic route for agmatine. Urea and 4-guanidinobutyrate were detected in the spent medium during growth on agmatine. The agmatine-grownA. nigermycelia contained significant levels of amine oxidase, 4-guanidinobutyraldehyde dehydrogenase, 4-guanidinobutyrase (GBase), and succinic semialdehyde dehydrogenase, but no agmatinase activity was detected. Taken together, the results support a novel route for agmatine utilization inA. niger. The catabolism of agmatine by way of 4-guanidinobutyrate to 4-aminobutyrate into the Krebs cycle is the first report of such a pathway in any organism.A. nigerGBase peptide fragments were identified by tandem mass spectrometry analysis. The corresponding open reading frame from theA. nigerNCIM 565 genome was located and cloned. Subsequent expression of GBase in bothEscherichia coliandA. nigeralong with its disruption inA. nigerfunctionally defined the GBase locus (gbu) in theA. nigergenome.