Global Roles of Ssn6 in Tup1- and Nrg1-dependent Gene Regulation in the Fungal Pathogen,Candida albicans

Abstract

In budding yeast, Tup1 and Ssn6/Cyc8 form a corepressor that regulates a large number of genes. This Tup1-Ssn6 corepressor appears to be conserved from yeast to man. In the pathogenic fungus Candida albicans, Tup1 regulates cellular morphogenesis, phenotypic switching, and metabolism, but the role of Ssn6 remains unclear. We show that there are clear differences in the morphological and invasive phenotypes of C. albicans ssn6 and tup1 mutants. Unlike Tup1, Ssn6 depletion promoted morphological events reminiscent of phenotypic switching rather than filamentous growth. Transcript profiling revealed minimal overlap between the Ssn6 and Tup1 regulons. Hypha-specific genes, which are repressed by Tup1 and Nrg1, were not derepressed in ssn6 cells under the conditions studied. In contrast, the phase specific gene WH11 was derepressed in ssn6 cells, but not in tup1 or nrg1 cells. Hence Ssn6 and Tup1 play distinct roles in C. albicans. Nevertheless, both Ssn6 and Tup1 were required for the Nrg1-mediated repression of an artificial NRE promoter, and lexA-Nrg1 mediated repression in the C. albicans one-hybrid system. These observations are explained in models that are generally consistent with the Tup1-Ssn6 paradigm in budding yeast. INTRODUCTIONTranscriptional repression plays a key role in controlling the growth and differentiation of eukaryotic cells. The fundamental importance of negative transcriptional regulators is reflected in their conservation during eukaryotic evolution. For example, orthologues of the Ssn6 (Cyc8) and Tup1 proteins, which act as global repressors in Saccharomyces cerevisiae, have been identified in humans, flies, worms, slime molds, and fungi (reviewed by Smith and Johnson, 2000).Our understanding of the mechanisms of action of Ssn6 and Tup1 is based largely on studies in budding yeast. The S. cerevisiae paradigm suggests that Ssn6 and Tup1 interact physically to form a corepressor complex that actively represses the transcriptional machinery (Williams et al., 1991;Redd et al., 1997;Gounalaki et al., 2000). The relevance of this paradigm to eukaryotes in general is emphasized by the observation that yeast Ssn6 can interact with human Tup1-like proteins to mediate transcriptional repression in human cells (Grbavec et al., 1999).The Ssn6-Tup1 corepressor is thought to repress transcription in yeast by two main mechanisms. First, the corepressor is thought to remodel chromatin on yeast promoters by recruiting histone deacetylases to these promoters and by positioning nucleosomes via direct interactions with histone tails (Edmondson et al., 1996;Davie et al., 2002Davie et al., , 2003Zhang and Reese, 2004a). Second, Tup1 is thought to interact directly with the transcriptional machinery to attenuate its activity (Carlson, 1997;Redd et al., 1997;Gromoller and Lehming, 2000;Papamichos-Chronakis et al., 2000). Recent experiments suggest that both mechanisms operate in yeast, but that they are utilized differentially to regulate distinct sets of yeast genes (Green and Johnson, 2004). However, both mechan...

The Yak1 Kinase Is Involved in the Initiation and Maintenance of Hyphal Growth inCandida albicans

“…Therefore, it is tempting to speculate that Yak1 may modify the activity of the Tup1-Nrg1, Tup1-Rfg1, or chromatin remodeling complexes in response to hypha-inducing signals and consequently allow derepression or activation to take place. A large fraction of the genes that are hypha-induced in a Yak1-dependent manner have been shown to be repressed by Tup1 Garcia-Sanchez et al, 2005), suggesting that Yak1 may interfere with Tup1-mediated repression. This hypothesis is supported by our observation that inactivation of the TUP1 gene can bypass the Yak1 requirement for filamentation (Figure 7), suggesting that Yak1 might function as a repressor of Tup1.…”

“…b ϩ indicates Tup1-negative regulation as inferred from transcript profiling data available in Garcia-Sanchez et al (2005) and Kadosh and Johnson (2005 Table 3). For these latter genes, we observed that 1) the activation ratio was always lower in the mutant strain than in the wild-type strain and 2) the level of expression in wild-type yeast cells was higher than that in yeast cells of the yak1⌬/yak1⌬ mutant strain (Table 3).…”

Regulatory Circuitry Governing Fungal Development, Drug Resistance, and Disease

“…Furthermore, NRG1 itself is downregulated during the yeast-to-hypha transition (71). Transcriptional profiling suggests that Nrg1 is also involved in the negative regulation of filament-specific transcripts but only targets Tup1 to a subset of promoters in the C. albicans genome (199,262,406,407). Nrg1 also has an important role in biofilm formation, where high levels of NRG1 inhibit yeast-tohypha morphogenesis yet still produce yeast cells that incorporate into biofilms.…”

Candida albicansCell Wall Proteins

“…Although transcriptional profiling reveals the conditions or transcriptional factors that regulate the expression of a protein, the function of the protein may remain unknown. For example, Rbe1p is a predicted secreted protein that is negatively regulated by Ace1p (256), Rim101p (23), Ssn6p (120), and Efg1p (356). Secreted proteins with enzymatic functions may catalyze reactions in which cell wall components are substrates, particularly glucan and chitin or other substrates, as in the case of trehalase.…”