Aspects of physiology of Histoplasma capsulatum (A review)

Yeast and mycelial forms of several strains of Histoplasma capsulatum have been analysed with respect to their ability to grow on a defined medium with or without the amino acid supplement. It appeared that whereas mycelial cells of all strains tested were prototrophic, the yeast cells of most strains stringently required L-cysteine for growth. This was due to the absence from these cells of an active form of an enzyme, sulfite reductase, normally needed for cysteine biosynthesis. We have found that the yeast cells of two strains (Downs and G 184 B) can grow without cysteine supplement if L-serine is added to the medium. These cells have an active sulfite reductase but the enzyme disappears when cysteine is added. Thus, the regulation of sulfite reductase is different in mycelium and yeast — the enzyme is constitutive or repressible, respectively.Examination of RNA synthetic components of H. capsulatum revealed that the major proportion of RNA polymerase of the yeast form is sensitive to inhibition by -amanitin. The sensitivity to the toxin disappears completely upon conversion to mycelial phase. The yeast cells possess an unusual enzyme capable of synthesizing oligonucleotides without the aid of a DNA template. The enzyme stimulates DNA synthesis in the reaction catalyzed by DNA polymerase from H. capsulatum or Escherichia coli. The above data are discussed in terms of regulatory mechanisms involved in the process of morphological conversion. It is proposed that efforts be directed toward the identification and isolation of specific gene products so that qualitative and quantitative analysis of the conversion could be carried out.presented, in part, at the 1st International Histoplasmosis Conference, held on April 10–12, 1978 in Atlanta, Georgia, U.S.A.