Catalyzes the two-electron reduction of urocanate to dihydrourocanate (also named imidazole propionate or deamino-histidine). The physiological electron donor is unknown; it might be the membrane-bound tetraheme cytochrome c (CymA). Enables anaerobic growth with urocanate as a sole terminal electron acceptor, and thus can provide the cells with a niche where no other bacteria can compete and survive. Is unable to reduce cinnamate and other unsaturated organic acids such as acrylic, crotonic, fumaric and orotic acids. Has no fumarate reductase or succinate dehydrogenase activity. A + dihydrourocanate = AH2 + urocanate Binds 1 FAD per subunit. Binds 1 FMN covalently per subunit. KM for urocanate is inferior to 10 uM and kcat is 360 sec(-1) (at pH 7.0). Is associated, but not too tight, with the membrane. By urocanate under anaerobic conditions. In the urdA-deficient strain, the urocanate reductase activity measured in the crude extract from cells anaerobically grown on a mixture of urocanate and fumarate (or urocanate and nitrate) is about 20-fold lower than in the wild-type. This strain is not affected in its ability to grow in the presence of fumarate, but fails to grow anaerobically on urocanate. Belongs to the FAD-dependent oxidoreductase 2 family. FRD/SDH subfamily.