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Eukaryotic Cell, June 2002, p. 353-365, Vol. 1, No. 3
1535-9778/02/$04.00+0     DOI: 10.1128/EC.1.3.353-365.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Roles of TUP1 in Switching, Phase Maintenance, and Phase-Specific Gene Expression in Candida albicans

Rui Zhao, Shawn R. Lockhart, Karla Daniels, and David R. Soll*

Department of Biological Sciences, University of Iowa, Iowa City, Iowa 52242-1324

Received 27 September 2001/ Accepted 6 March 2002

Candida albicans strain WO-1 switches spontaneously and reversibly between a "white" and "opaque" phenotype that affects colony morphology, cellular phenotype, and expression of a number of phase-specific genes and virulence traits. To assess the role of the transcription regulator Tup1p in this phenotypic transition, both TUP1 alleles were deleted in the mutant {Delta}tup1. {Delta}tup1 formed "fuzzy large" colonies made up of cells growing exclusively in the filamentous form. {Delta}tup1 cells did not undergo the white-opaque transition, but it did switch spontaneously, at high frequency (~10-3), and unidirectionally through the following sequence of colony (and cellular) phenotypes: "fuzzy large" (primarily hyphae) -> "fuzzy small" (primarily pseudohyphae) -> "smooth" (primarily budding yeast) -> "revertant fuzzy" (primarily pseudohyphae). Northern analysis of white-phase, opaque-phase, and hypha-associated genes demonstrated that Tup1p also plays a role in the regulation of select phase-specific genes and that each variant in the {Delta}tup1 switching lineage differs in the level of expression of one or more phase-specific and/or hypha-associated genes. Using a rescued {Delta}tup1 strain, in which TUP1 was placed under the regulation of the inducible MET3 promoter, white- and opaque-phase cells were individually subjected to a regime in which TUP1 was first downregulated and then upregulated. The results of this experiment demonstrated that (i) downregulation of TUP1 led to exclusive filamentous growth in both originally white- and opaque-phase cells; (ii) the white-phase-specific gene WH11 continued to be expressed in TUP1 downregulated cultures originating from white-phase cells, while WH11 expression remained repressed in TUP1-downregulated cultures originating from opaque-phase cells, suggesting that cells maintained phase identity in the absence of TUP1 expression; and (iii) subsequent upregulation of TUP1 resulted in mass conversion of originally white-phase cells to the opaque phase and maintenance of originally opaque-phase cells in the opaque phase and in the resumption in both cases of switching, suggesting that TUP1 reexpression turns on the switching system in the opaque phase.

* Corresponding author. Mailing address: 302 Biology Bldg. East, University of Iowa, Iowa City, IA 52242-1324. Phone: (319) 335-1111. Fax: (319) 335-2772. E-mail: david-soll{at}uiowa.edu.

Eukaryotic Cell, June 2002, p. 353-365, Vol. 1, No. 3
1535-9778/02/$04.00+0     DOI: 10.1128/EC.1.3.353-365.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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Copyright © 2002 by the American Society for Microbiology.