Coordinate regulation of enzyme inducibility and developmental competence in Aspergillus nidulans

Abstract

Spores of the fungus Aspergillus nidulans germinated in defined liquid medium and produced hyphae which grew as submerged undifferentiated cultures. The ability of cultures of various ages to respond to enzyme inductive conditions was determined. Intracellular nitrate reductase was induced by addition of nitrate to defined growth medium; extracellular neutral protease was induced by removal of sulfur. These enzymes are metabolically unrelated, and not required for growth or differentiation. At 23–26 hr after inoculation of spores, growing cultures underwent a distinct decrease in their induction response for each inducible enzyme. Growing cultures maintained the activity of the constitutive enzyme glucose-6-phosphate dehydrogenase. The change in the ability of cultures to respond to biochemical inductive stimuli occurred at the same time that cultures became developmentally competent to respond to a morphological inductive stimulus. The concurrent changes in the ability of cultures to respond to enzymatic and morphological induction was not due to changes in growth rate, depletion of medium, inhibitor production, or to an inability to maintain enzyme activities. The changes were independent of any specific induction stimulus. The mechanism responsible for the pleiotropic effects appears to be endogenously controlled. These results suggest the existence in eukaryotes of a mechanism for the coordinated regulation of potential to express facultative functions.