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The Neutral and Alkaline Proteases of Aspergillus nidulans
Abstract
SUMMARY: When grown on glucose salts medium containing protein as sole nitrogen source, Aspergillus nidulans strain biA1 synthesizes three neutral or alkaline extracellular proteases (α, γ, e) and one protease (β) that is strictly intracellular. The four proteases are individually distinguishable by electrophoresis, inhibitor-sensitivity, substrate-preference and pH optimum tests. When ammonium ion (50 mM) is added to the medium, protease β is synthesized as two precursors (β1 and β2) and synthesis of α, γ and e is repressed. The precursors β1 and β2 can be separated by electrophoresis; they are weakly active at 50 °C and have no detectable activity in a protease assay. Conversion of β1 and β2 to β takes place during storage of mycelial extracts at 4 °C; it is accompanied by the appearance of detectable activity in the protease assay. The intracellular enzyme γ also occurs as an (active) precursor, δ. Conversion of δ to γ takes place both in vivo and in stored mycelial extracts, but does not occur in stored culture filtrates.
A recessive, single-gene mutation (xprC1), results in simultaneous loss of all extracellular proteases but xprC1 strains grow and differentiate normally on medium containing non-protein nitrogen. Since the extracellular proteases are absent, both in xprC1, and in biA1 when grown in the presence of ammonium, it is concluded that they are not required for normal growth and differentiation. Also as xprC1 does not utilize protein as sole nitrogen or carbon source, protease β is not used in vivo to hydrolyse exogenous protein.
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© Society for General Microbiology 1973