Induction of the Candida albicans filamentous growth program by relief of transcriptional repression: a genome-wide analysis

Abstract

Candida albicans, the major human fungal pathogen, undergoes a reversible morphological transition from blastospores (round budding cells) to filaments (elongated cells attached end-to-end). This transition, which is induced upon exposure of C. albicans cells to a number of host conditions, including serum and body temperature (37 degrees C), is required for virulence. Using whole-genome DNA microarray analysis, we describe 61 genes that are significantly induced (> or =2-fold) during the blastospore to filament transition that takes place in response to exposure to serum and 37 degrees C. We next show that approximately half of these genes are transcriptionally repressed in the blastospore state by three transcriptional repressors, Rfg1, Nrg1, and Tup1. We conclude that the relief of this transcriptional repression plays a key role in bringing the C. albicans filamentous growth program into play, and we describe the framework of this transcriptional circuit.

Figure 1.

Morphology of cells undergoing the blastospore to filament transition. An overnight culture grown in YEPD medium at 30°C was diluted 1:10 into prewarmed YEPD medium at 30°C or at 37°C in the presence or absence of 10% FCS. Induction time (hrs.) is shown on top. Aliquots of cells were fixed in 4.5% formaldehyde, washed twice with 1 × phosphate-buffered saline, and then visualized by Nomarski optics and photographed under 100× magnification. Please note that the 0-h time point photograph shows cells immediately before induction.

Figure 2.

Transcriptional profile of most serum- and temperature-induced genes. Fold induction, relative to the zero time point, is shown for four serum- and temperature-induced genes (HYR1, ALS3, HWP1, SAP5) when cells were grown in YEPD medium, at 30 or 37°C in the presence or absence of 10% FCS, at the indicated time points. ^*Please note that SAP5 expression levels may reflect, in part, SAP4 or SAP6 expression levels due cross-hybridization on the microarray, because these gene family members are nearly identical at the DNA level.

Figure 3.

Cluster diagram of the top serum- and temperature-induced genes (≥5-fold mean induction, n = 2, at the 37°C + 10% FCS 1-h time point) with corresponding rfg1, nrg1, and tup1 mutant data. Only data from one serum- and temperature-induction experiment and only genes with greater than 89% of data present are shown. Each column represents data from a single microarray experiment. Red, increased expression; green, reduced expression; gray, no data available. ^*Please note that SAP5 expression levels may reflect, in part, SAP4 or SAP6 expression levels due cross-hybridization on the microarray, because these gene family members are nearly identical at the DNA level.

Figure 4.

Cluster diagram of genes showing a median elevated expression ≥5-fold (n = 6) in the nrg1 mutant with corresponding tup1 mutant data (n = 6). Only genes with greater than 66% of data present are shown. Each column represents data from a single microarray experiment. Red, increased expression; gray, no data available.

Figure 5.

Representation of gene classes induced during the blastospore to filament transition compared with the genome as a whole. Asterisks indicate gene classes which are overrepresented in the serum- and temperature-induced gene set compared with the genome as a whole. Gene classification was based on S. cerevisiae gene ontology (GO) terms (Dwight et al., 2002; Harris et al., 2004). Please note that the “unknown function” gene class includes all genes of unknown function (with and without S. cerevisiae homologues or reciprocal hits).

Figure 6.

A model for control of genes induced during the blastospore to filament transition by the Rfg1, Nrg1, and Tup1 pathways. Rfg1, Nrg1, and at least one additional DNA-binding protein (X) bind to the promoters of filament- and virulence-specific target genes and direct transcriptional repression via recruitment of the Tup1 corepressor. The Nrg1 transcript itself has previously been shown to be down-regulated in the presence of 10% serum and temperature (Braun et al., 2001; Murad et al., 2001). Rfg1 repression activity is proposed to be regulated by filament-inducing conditions at the posttranslational level (Kadosh and Johnson, 2001). Target genes are indicated in boxed areas. ^*Cell wall components; ⁺ER/Golgi/secretion genes.