Phenotypic switching and mating in the pathogenic yeast Candida albicans.

摘要

Candida albicans is the most pervasive human fungal pathogen. The success of this pathogen stems in part from phenotypic plasticity, which is provided by several developmental programs. The research reported in this thesis has focused on the basic biology and molecular mechanisms of two closely related developmental programs, white-opaque switching and mating. Disruption of the transcription repressor TUP1 in strain WO-1 abolished white-opaque switching, but not phenotypic switching in general. Cells of the tup1 mutant grew primarily in the filamentous form. Rescuing the tup1 mutant by a controllable MET3 promoter revealed that TUP1 was not required for phase maintenance. Re-expression of TUP1 in tup1 mutant cells led to mass conversion to the opaque phenotype, suggesting TUP1 was involved in the switch event. The repressor complex Mtla1p-Mtlalpha2p was reconstructed in MTL -homozygotes. It blocked white-opaque switching and mating. By selectively placing MTLa1 expression under control of an opaque promoter, it was demonstrated that the Mtla1p-Mtlalpha2p complex suppresses maintenance or downstream expression of the opaque phenotype. Microarray analyses was used to monitor global gene expression patterns during C. albicans mating. Fifty genes were found to be up-regulated and thirty down-regulated. Combined with northern analysis, a unique group of OMUR genes, the expression of which is constitutive in the exponential phase of white and opaque cells, but selectively down-regulated in the stationary phase of opaque cells, was found to be induced during mating by pheromone. A new method for measuring nuclear DNA content by quantitative fluorescence microscopy was developed. It was used to demonstrate that cells forming conjugation tubes contained only un-replicated DNA, demonstrating that the G1 phase of the cell cycle was required for shmooing. Expression of the OMUR genes, however, proved independent of the cell cycle, suggesting that selective down-regulation of these genes in stationary phase opaque cells is not required for the acquisition of mating competency.