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A Possible Role for Urease as a Storage Protein in Aspergillus tamarii

J. W. ZAWADA and J. F. SUTCLIFFE
Annals of Botany
Vol. 48, No. 6 (December 1981), pp. 797-810
Published by: Oxford University Press
Stable URL: http://www.jstor.org/stable/42756738
Page Count: 14
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A Possible Role for Urease as a Storage Protein in Aspergillus tamarii
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Abstract

A marked decrease in mycelial urease activity during the endogenous phase of undifferentiated Aspergillus tamarii cultures was found to be independent of preparative procedures but related to the depletion of external nutrients. The enzyme, which was synthesized during the active growth stage, was produced in similar quantities with ammonium or urea as sole nitrogen source and at its peak represented c. 8·5 per cent of the total soluble protein pool of the mycelium. It was found to show maximum activity at pH 8·20-8·65 when measured in cell-free, phosphate-buffered extracts. Isolation of urease from different stages of the endogenous phase by affinity chromatography has shown that the observed decrease in activity was due to breakdown of the enzyme protein in mature cultures, followed by the progressive deactivation of residual enzyme during the autolytic stage. Since selective inhibition of 80-90 per cent of activity by acetohydroxamic acid in media containing urea as the only nitrogen source or total repression of urease synthesis by L-histidine in ammonium-grown cultures did not interfere with normal growth, it was concluded that in A. tamarii urease fulfils the function of a storage protein with a measure of catalytic activity.

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