Cysteine Dioxygenase Enzyme Activity and Gene Expression in the Dimorphic Pathogenic Fungus Histoplasma capsulatum Is in both the Mold and Yeast Morphotypes and Exhibits Substantial Strain Variation

In the dimorphism (mold/yeast) Histoplasma capsulatum (Hc) literature are reports that yeast (the so-called pathogenic form) uniquely expresses a cysteine dioxygenase (CDO, approx. 10,500 dal) activity which the mold morphotype (the so-called saprophytic soil form) does not express (C.F., Kumar et al., Biochem 22, 762, 1983). This yeast-specific CDO activity is postulated to play a critical role in the mold-to-yeast shift. A number of years ago, our lab isolated the gene encoding the Hc cysteine dioxygenase (CDO1, Genbank accession AY804144) and noted significant expression in the mold morphotype of several Histoplasma strains and also determined that the predicted protein would be over double the 10,500 dal reported by Kumar et al. Our report demonstrates (in the class 1 Downs strain, the class 2 G271B strain and two Panamanian strains, 184AS and 186AS) that the CDO1 gene is expressed in both the mold and yeast morphotypes and both morphotypes show significant CDO activity. Furthermore, we show via a FLAG-tag analysis that the expressed protein is approximately 24.7 ± 2.4 kd, in agreement with the putative protein sequence (determined from cDNA sequence) which yields 23.8 kd and is consistent with most other eukaryotic CDO enzymes. Additionally, we demonstrate that intracellular cysteine levels are actually significantly higher in the mold form of the two Panamanian strains, 184AS and 186AS, equal in both mold and yeast in the class 1 Downs strain and significantly higher in yeast of the more pathogenic class 2 G217B strain.