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Updated: Aug 22, 2026
Biolistic Transformation of a Fluorescent Tagged Gene into the Opportunistic Fungal Pathogen Cryptococcus neoformans
Published on: March 19, 2015
Cryptococcus neoformans methionine synthase: expression analysis and requirement for virulence
Renata C Pascon1, Tonya M Ganous1, Joanne M Kingsbury1
1Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, NC 27710, USA.
Abstract:
This paper describes (i) the expression profile of the methionine synthase gene (MET6) in the human pathogenic fungus Cryptococcus neoformans and (ii) the phenotypes of a C. neoformans met6 mutant. In contrast to the MET3 gene, which showed no significant change in expression in any environmental condition tested, the MET6 gene showed a substantial induction in response to methionine and a dramatic transcriptional induction in response to homocysteine. Like a met3 mutant, the met6 mutant was a methionine auxotroph. However, relative to a met3 mutant, the met6 mutant grew very slowly and was less heat-shock resistant. In contrast to a met3 mutant, the met6 mutant lost viability when starved of methionine, and it was deficient in capsule formation. Like a met3 mutant, the met6 mutant was avirulent. In contrast to a met3 mutant, the met6 mutant was hypersensitive to fluconazole and to the calcineurin inhibitors FK506 and cyclosporin A. A synergistic fungicidal effect was also found between each of these drugs and met6. The phenotypic differences between the met3 and met6 mutants may be due to the accumulation in met6 mutants of homocysteine, a toxic metabolic intermediate that inhibits sterol biosynthesis.
Insights
The methionine synthase gene (MET6) in Cryptococcus neoformans is induced by methionine and homocysteine. A met6 mutant shows severe growth defects and increased drug sensitivity, impacting virulence.
Area of Science:
- Mycology
- Molecular Biology
- Medical Mycology
Background:
- Cryptococcus neoformans is a human pathogenic fungus.
- Methionine biosynthesis is crucial for fungal survival and virulence.
- The MET6 gene encodes methionine synthase, a key enzyme in this pathway.
Purpose of the Study:
- To investigate the expression profile of the MET6 gene in C. neoformans.
- To characterize the phenotypic traits of a C. neoformans met6 mutant.
- To compare the met6 mutant phenotype with that of a previously studied met3 mutant.
Main Methods:
- Gene expression analysis of MET6 under various conditions.
- Phenotypic characterization of a C. neoformans met6 mutant, including growth, heat shock resistance, viability, capsule formation, and virulence.
- Antifungal drug susceptibility testing of the met6 mutant.
Main Results:
- MET6 expression is induced by methionine and homocysteine.
- The met6 mutant is a methionine auxotroph with slow growth, reduced heat shock resistance, and loss of viability upon methionine starvation.
- The met6 mutant is avirulent and hypersensitive to fluconazole and calcineurin inhibitors, exhibiting synergistic effects with these drugs.
Conclusions:
- MET6 plays a critical role in C. neoformans growth, survival, and virulence.
- The accumulation of homocysteine in met6 mutants may contribute to observed phenotypic differences and drug hypersensitivity.
- Targeting MET6 or related pathways could be a strategy for antifungal therapy.
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