ABSTRACT
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 Δ
tup1
. Δ
tup1
formed “fuzzy large” colonies made up of cells growing exclusively in the filamentous form. Δ
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 Δ
tup1
switching lineage differs in the level of expression of one or more phase-specific and/or hypha-associated genes. Using a rescued Δ
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.
