PROBING THE YEAST PHASE-SPECIFIC EXPRESSION OF THE CBP1 GENE IN HISTOPLASMA-CAPSULATUM

Histoplasma capsulatum is a pathogenic fungus that exists in two disti nct forms. The saprophytic mycelial phase inhabits moist soil environm ents; once inhaled, hyphae and conidia convert to a unicellular yeast phase that is capable of parasitizing macrophage phagolysosomes. Yeast s cultures, but not mycelial cultures, release large quantities of a c alcium-binding protein (CBP) which may be important in calcium acquisi tion during intracellular parasitism, In this study, we show that the gene encoding CBP (CBP1) is transcriptionally regulated. To identify p romoter sequences that are important for yeast phase-specific activity , we created a series of fusions between successively truncated CBP1 5 ' untranslated regulatory sequences and the Eschericha coli lacZ gene. The fusions were constructed on a telomeric shuttle plasmid that can replicate autonomously in the fungus. By assaying for beta-galactosida se activity from H. capsulatum transformants, we identified a 102-bp r egion that mediates promoter activation and yeast phase promoter activ ity, Base pair substitution analysis suggests that the sequences betwe en 839 and 877 bp upstream of the start codon are the most important f or this positive regulation.