Control of the synthesis of a single enzyme by multiple regulatory circuits in Neurospora crassa

Neurospora crassa synthesizes and secretes an extracellular protease into its growth medium when an exogenous protein serves as its principal source of sulfur, nitrogen, or carbon. The enzymes produced under these three growth conditions have been compared by a number of criteria. The results indicate that the same extracellular protease with a molecular weight of 31,000 is synthesized during the three different metabolic conditions. A regulatory mutant, which lacks a positive signal required for the synthesis of a family of related enzymes for sulfur metabolism, cannot synthesize the protease in response to a limitation for sulfur; yet, this same mutant is capable of producing the enzyme when it is limited for either nitrogen or carbon. A second regulatory mutant, defective in the control of nitrogen metabolism, fails to synthesize the protease only when it is limited for nitrogen. The evidence suggests that a single structural gene for this extracellular protease exists and that it is regulated in a complex fashion such that control signals arising from any one of the three distinct regulatory circuits can activate it for expression. A model is proposed for complex regulation of the synthesis of this enzyme.