Catalyzes the hydroxylation of urate to 5-hydroxyisourate (HIU). Is involved in the urate degradation pathway to allantoin. H(+) + NADH + O2 + urate = 5-hydroxyisourate + H2O + NAD(+) kcat is 23.6 sec(-1) for the NADH-dependent oxidation of urate. Exhibits a Michaelian behavior toward urate and NADPH and a cooperative behavior toward NADH. Purine metabolism; urate degradation. Homodimer. Cells lacking this gene are auxotrophic for urate. Growth defect can be complemented by introduction of XCC0279 from X.campestris or Mvan_5278 from M.vanbaalenii. Belongs to the FAD-dependent urate hydroxylase family.