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Abstract
Cryptococcus neoformans is a ubiquitous environmental fungus and opportunistic human fungal pathogen. Due to the range of environments this fungus can be exposed to, C. neoformans has many mechanisms in place to tolerate environmental stressors. The plasma membrane is the primary site of response to extracellular stimuli. The lipids and proteins that compose the plasma membrane can be rearranged based on the needs of the cell. Lipids in particular are important for processes such as prospore formation and endocytosis in the yeast Saccharomyces cerevisiae. I found that the spores of C. neoformans are enriched in ergosterol, the main fungal sterol, and that deletion of the transcription factor involved in regulating ergosterol biosynthesis (SRE1) was defective at producing spores. We determined that overexpression of ergosterol biosynthesis genes in this deletion mutant restored sporulation frequencies. I also found that nickel (Ni), a common soil contaminant, reduces cellular ergosterol content and that the SRE1 deletion mutant was extremely sensitive to media supplemented with Ni. The inability of sre1Δ to express ergosterol biosynthesis pathway (EBP) genes is likely the cause of the Ni sensitivity phenotype. I found that overexpression of ERG25, a component of the EBP, was capable of rescuing the sre1Δ phenotype. Erg25 is known to bind iron and site directed mutagenesis of the histidine residues predicted to interact with the iron ion abolished the ability of the ERG25 overexpression strain to rescue the growth defect of sre1Δ on Ni. The reduction of sterols by Ni is conserved in mammalian lung epithelial cells. The maintenance of ergosterol is critical for cell survival and understanding of the stimuli and/or cellular processes that cause changes in ergosterol are important for understanding the biology of this fungus as a whole.