[Repression of urease biosynthesis in Neurospora crassa by ammonium ions]

The regulation of the synthesis of the enzyme urease (urea amido hydrolase E.C. 3.5.1.5.) in Neurospora crassa was investigated. The biosynthesis of urease is repressed by ammonium ions. Under ammonium excess conditions the specific activity of urease decreases from 0.980 to 0.180 mumoles NH3/min/mg protein. By addition of cycloheximide it was shown that ammonia influences the synthesis of this enzyme. Enzyme induction by the substrate could be excluded. Even under the conditions of highest repression a specific activity of urease of 0.180 mumoles NH3/min/mg protein was measured. Possible causes of this constitutive enzyme level are discussed.