Streptomyces aspartate transcarbamoylase is a dodecamer with dihydroorotase activity.

Aspartate transcarbamoylase (ATCase) was purified from Streptomyces griseus. The enzyme is a dodecamer with a molecular mass of approximately 450 kDa. The holoenzyme is a complex of ATCase and active dihydroorotase (DHOase) subunits. The ATCase and DHOase activities co-purify after gel filtration and ion-exchange chromatography. Denaturing gel electrophoresis separates the holoenzyme into a 38-kDa ATCase polypeptide and a 47-kDa DHOase polypeptide. The holoenzyme retained ATCase and DHOase activity after being heated to 65 degrees C for 5 min, but after storage at 4 degrees C for 24 hours lost ATCase activity. Previously, the Pseudomonas putida Class A ATCase was defined by Schurr et al. (J Bacteriol 177, 1751-1759) as requiring an inactive DHOase to be functional. Here, we show that an active DHOase is part of the dodecameric ATCase/DHOase complex in Streptomyces. To distinguish those Class A ATCases with active DHOases from those with degenerate DHOases, we suggest the subdivision, Class A(1), for the former and Class A(2) for the latter.