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Cysteine biosynthesis in a fungus, Histoplasma capsulatum

D A Stetler, G Boguslawski

    Sabouraudia
    |March 1, 1979
    PubMed

    Insights

    Histoplasma capsulatum yeast cells require -SH compounds for growth due to inactive sulfite reductase. The Downs strain

    Area of Science:

    • Medical Mycology
    • Biochemistry
    • Fungal Pathogenesis

    Background:

    • Cysteine biosynthesis is essential for fungal growth.
    • Dimorphic fungi like Histoplasma capsulatum exhibit distinct morphological phases with potentially different metabolic requirements.
    • Understanding metabolic pathways in pathogenic fungi is crucial for developing targeted therapies.

    Purpose of the Study:

    • To investigate the cysteine biosynthesis pathway in different strains of Histoplasma capsulatum.
    • To identify the metabolic basis for auxotrophy in the yeast phase of H. capsulatum.
    • To characterize the regulation of sulfite reductase in H. capsulatum.

    Main Methods:

    • Analysis of cysteine biosynthesis in multiple H. capsulatum strains.
    • Growth assays in the presence and absence of -SH compounds and amino acids.
    • Enzyme activity assays for sulfite reductase.
    • Polyacrylamide gel electrophoresis (PAGE) of cellular extracts.

    Main Results:

    • Mycelial H. capsulatum cells are prototrophic, while most yeast cells are auxotrophic for -SH compounds.
    • The auxotrophy is linked to a lack of active sulfite reductase in the yeast phase.
    • The Downs strain's yeast phase has active sulfite reductase but a different metabolic block, allowing growth with serine.
    • Sulfite reductase in the Downs yeast form is repressed by cysteine, unlike the constitutive mycelial form.
    • A specific protein band associated with sulfite reductase activity was detected in yeast cells grown in minimal or serine media.

    Conclusions:

    • A critical defect in sulfite reductase activity in the yeast phase explains the cysteine auxotrophy in most H. capsulatum strains.
    • The Downs strain presents a unique metabolic regulation of cysteine biosynthesis, with a distinct block and cysteine-mediated repression of sulfite reductase.
    • Differential expression and regulation of sulfite reductase between yeast and mycelial phases are key to understanding H. capsulatum's dimorphism and metabolic adaptability.

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