This study examines the regulation of extracellular carboxyl proteinase secretion by Rhizopus oligosporus, a fungus used in tempe production from soy beans. The researchers transferred mycelium from nutrient-rich minimal medium to media deficient in nitrogen, sulfur, or carbon. Proteinase secretion was repressed in minimal medium but derepressed when any single nutrient was limited. Nitrogen-metabolite repression operated at the transcriptional level, requiring both RNA and protein synthesis. Unlike some other fungi, R. oligosporus showed no induction of proteinase secretion by exogenous protein in any medium tested, even at high concentrations (1% w/v). Similar derepression patterns were observed for sulfur and carbon limitation, though carbon limitation showed weaker derepression and required low ionic strength medium. Analysis of the proteinase gene sequence revealed putative binding sites for regulatory proteins (NIT2 and CYS3) similar to those controlling nutrient-responsive genes in Neurospora crassa and Aspergillus nidulans, suggesting analogous regulatory mechanisms across fungi.

Key findings

• Proteinase secretion in R. oligosporus is repressed by low-molecular-mass sources of nitrogen, sulfur, and carbon; secretion occurs when any of these nutrients become deficient
• Nitrogen-metabolite repression operates at the transcriptional level, with both RNA and protein synthesis required for proteinase secretion
• Unlike N. crassa and M. miehei, exogenous protein does not induce proteinase secretion in R. oligosporus under any conditions tested
• The proteinase gene contains putative regulatory binding sites (NIT2 and CYS3) analogous to nutrient-sensing regulatory elements in other fungi
• Distinct regulatory circuits control proteinase secretion in response to limitation of each nutrient (nitrogen, sulfur, and carbon)