Selection and characterization of ura5 mutants of Histoplasma capsulatum

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Abstract
The combined use of non-aggregating Histoplasma capsulatum strains and a defined medium which allows quantitative plating of the yeast phase has allowed us to select 5-fluoroorotic acid (5-FOA)-resistant mutants of this dimorphic fungus. Approximately two-thirds of the 5-FOA-resistant strains were auxotrophic for uracil; all were deficient in orotidine-5′-monophosphate pyrophosphorylase (OMPpase) activity. One class of OMPpase mutant (α), which retained a low level of OMPpase activity, was auxotrophic in the yeast phase (37°C) but grew slowly in the mycelial phase (25°C) without exogenous uracil. This phenotype was not due to a temperature-sensitive OMPpase activity. Both wild-type and α mutants had a higher OMPpase activity in the mycelial phase than the yeast phase; this increased activity may be sufficient to allow mycelial growth of α mutants.