Abstract
When the mycelial to yeast transition of the dimorphic fungus Histoplasma capsulatum is induced by a temperature shift from 25 to 37 degrees C, the activities of the cytochrome system and the alternate oxidase decrease in parallel over the first 24 to 40 h (stage 1 of the transition). The decrease in activity of the cytochrome system is correlated with extensive decreases in the amounts of cytochromes b, c, and aa3, assayed spectrophotometrically. After 40 h, the cells enter a dormant phase (stage 2 of the transition) and cysteine or other sulfhydryl-containing compounds are required to reactivate mitochondrial respiration. This reactivation is due to the establishment of shunt pathways which bypass blocked segments of the electron transport system. The "shunt" pathways operate normally in mycelia grown at 25 degrees C, but are shut down during the transition, possibly because of depletion of intracellular cysteine. The longstanding observation that cysteine is required to progress beyond the initial stages of the morphological transition may be due, at least in part, to the reactivation of these "shunt" pathways.
Highlights
When the mycelial toyeast transition of the di- cyanide; andtheother,anunidentifiedalternate oxidase morphic fungus Histoplasma capsulatumis induced by which isinsensitivetoantimycinand cyanide, butcan be a temperature shift from 25 to 37 “C, the activities of specificially inhibited by salicylhydroxamic acid
The flasks were harvested at different times, and respiration rates were measured
Cell respiration rates were expressed as microliters of 0, per h per mg of cells at 37 "C for yeast and 25 "C for mycelium
Summary
Culture Conditions-The wild type Downs strain of H. capsulatum, matingtype (-), was used in this study. The flasks were harvested at different times, and respiration rates were measured. The mitochondrial pellets were resuspended in mitochondrial respiration buffer consisting of 0.3 M sucrose, 8 mM NaH2P04,5 mM MgCI2,1mM ethylenediaminetetraacetic acid, 8 mM Tris-CI, pH 7.2. Measurements of Oxygen Consumption-Cells were suspended in cell respiration buffer containing 1% mannose, 1 mMCaC12, and 1 mM dimethylglutaric acid (pH 7.2). Cell respiration rates were expressed as microliters of 0, per h per mg of cells (dry weight) at 37 "C for yeast and 25 "C for mycelium. Oxygen consumption by mitochondria was measured at 25 and 37 "C in a chamber containing the mitochondrial respiration buffer saturated with air. In theabsence of cells or mitochondria, gave no detectable oxygen consumption in either thecell or the mitochondrial respiration buffer. Protein Measurements-Protein was measured by the Lowry method [8]
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