Nitrogen regulation of uricase synthesis in Neurospora crassa

Summary

Neurospora crassa can utilize various purine bases as a nitrogen source. Synthesis of the purine catabolic enzymes such as uricase requires induction by the intermediate uric acid and is also subject to nitrogen catabolic repression. Uricase synthesis has been studied in more detail to gain additional insight into this control circuit. The synthesis of uricase was repressed when wild-type Neurospora cells were grown in medium containing ammonium salts. The am mutant, which lacks NADP-linked glutamate dehydrogenase activity, was found to synthesize uricase even in the presence of ammonia. However, repression of uricase synthesis was restored in am by glutamate. The am mutant was shown to transport the methylammonium ion at a normal rate with K_m and V_max values which were indistinguishable from those of wild-type. Similarly, unlike the situation in wild-type, ammonia or glutamate failed to repress uricase synthesis in the gln mutant, which lacks glutamine synthetase activity, although glutamine was fully effective as a repressor in gln. These results suggest that neither ammonium per se nor glutamate are themselves nitrogen corepressors, but rather that glutamine has this role in Neurospora.

The induction of uricase synthesis was found to be blocked by lomofungin, an inhibitor of RNA synthesis, and by cycloheximide, a protein synthesis inhibitor. Uricase messenger RNA could be accumulated during induction conditions in the presence of cycloheximide, and subsequently translated to yield active enzyme. These results suggest that the regulation of uricase synthesis occurs at the transcriptional level. A time lag of nearly 120 min was found during induction before an increase occurred in the specific activity of uricase. Following induction, uricase messenger RNA could not be detected (by its subsequent translation in vivo) until after 80 to 90 minutes. The functional half-life of uricase messenger RNA was estimated to be 25±3 minutes.