An Iron Homeostasis Regulatory Circuit with Reciprocal Roles in Candida albicans Commensalism and Pathogenesis

Department of Medicine, Division of Infectious Diseases, University of California San Francisco, USA.
Cell host & microbe (Impact Factor: 12.19). 08/2011; 10(2):118-35. DOI: 10.1016/j.chom.2011.07.005
Source: PubMed

ABSTRACT The mammalian gastrointestinal tract and bloodstream are highly disparate biological niches that differ in concentrations of nutrients such as iron. However, some commensal-pathogenic microorganisms, such as the yeast Candida albicans, thrive in both environments. We report the evolution of a transcription circuit in C. albicans that controls iron uptake and determines its fitness in both niches. Our analysis of DNA-binding proteins that regulate iron uptake by this organism suggests the evolutionary intercalation of a transcriptional activator called Sef1 between two broadly conserved iron-responsive transcriptional repressors, Sfu1 and Hap43. Sef1 activates iron-uptake genes and promotes virulence in a mouse model of bloodstream infection, whereas Sfu1 represses iron-uptake genes and is dispensable for virulence but promotes gastrointestinal commensalism. Thus, C. albicans can alternate between genetic programs conferring resistance to iron depletion in the bloodstream versus iron toxicity in the gut, and this may represent a fundamental attribute of gastrointestinal commensal-pathogens.


Available from: Kalyan Pande, Apr 18, 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Iron scavenging constitutes a crucial challenge for survival of pathogenic microorganisms in the iron-poor host environment. Candida albicans, like many microbial pathogens, is able to utilize iron from hemoglobin, the largest iron pool in the host's body. Rbt5 is an extracellular glycosylphosphatidylinositol (GPI)-anchored heme-binding protein of the CFEM family that facilitates heme-iron uptake by an unknown mechanism. Here, we characterize an additional C. albicans CFEM protein gene, PGA7, deletion of which elicits a more severe heme-iron utilization phenotype than deletion of RBT5. The virulence of the pga7-/- mutant is reduced in a mouse model of systemic infection, consistent with a requirement for heme-iron utilization for C. albicans pathogenicity. The Pga7 and Rbt5 proteins exhibit distinct cell wall attachment, and discrete localization within the cell envelope, with Rbt5 being more exposed than Pga7. Both proteins are shown here to efficiently extract heme from hemoglobin. Surprisingly, while Pga7 has a higher affinity for heme in vitro, we find that heme transfer can occur bi-directionally between Pga7 and Rbt5, supporting a model in which they cooperate in a heme-acquisition relay. Together, our data delineate the roles of Pga7 and Rbt5 in a cell surface protein network that transfers heme from extracellular hemoglobin to the endocytic pathway, and provide a paradigm for how receptors embedded in the cell wall matrix can mediate nutrient uptake across the fungal cell envelope.
    PLoS Pathogens 10/2014; 10(10):e1004407. DOI:10.1371/journal.ppat.1004407 · 8.14 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The heterotrimeric CCAAT-binding complex (CBC) is evolutionary conserved in eukaryotic organisms including fungi, plants and mammals. The CBC consists of three subunits, which are named in the filamentous fungus Aspergillus nidulans HapB, HapC and HapE. HapX, a fourth CBC subunit was identified exclusively in fungi, except of Saccharomyces cerevisiae and closely related Saccharomycotina species. The CBC:HapX complex acts as the master regulator of iron homeostasis. HapX belongs to the class of basic region leucine zipper (bZIP) transcription factors. We demonstrated that the CBC and HapX bind cooperatively to bipartite DNA motifs with a general HapX:CBC:DNA 2:1:1 stoichiometry in a class of genes that are repressed by HapX:CBC in A. nidulans during iron limitation. This combinatorial binding mode requires protein-protein interaction between the N-terminal domain of HapE and the N-terminal CBC-binding domain of HapX as well as sequence-specific DNA-binding of both the CBC and HapX. Initial binding of the CBC to CCAAT boxes is mandatory for DNA recognition of HapX. HapX specifically targets the minimal motif 5'-GAT-3', which is located in a distance of 11 to 12 base pairs downstream of the respective CCAAT box. Single nucleotide substitutions at the 5'- and 3'-end of the GAT motif as well as different spacing between the CBC and HapX DNA-binding sites revealed a remarkable promiscuous DNA-recognition mode of HapX. This flexible DNA-binding code may have evolved as a mechanism for fine-tuning the transcriptional activity of CBC:HapX at distinct target promoters. Copyright © 2015, The American Society for Biochemistry and Molecular Biology.
    Journal of Biological Chemistry 01/2015; DOI:10.1074/jbc.M114.628677 · 4.60 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Flavohemoglobins are the main detoxifiers of nitric oxide (NO) in bacteria and fungi and are induced in response to nitrosative stress. In fungi, the flavohemoglobin encoding gene YHB1 is positively regulated by transcription factors which are activated upon NO exposure. In this study, we show that in the model yeast Saccharomyces cerevisiae and in the human pathogen Candida glabrata, the transcription factor Yap7 constitutively represses YHB1 by binding its promoter. Consequently, YAP7 deletion conferred high NO resistance to the cells. Co-immunoprecipitation experiments and mutant analyses indicated that Yap7 represses YHB1 by recruiting the transcriptional repressor Tup1. In S. cerevisiae, YHB1 repression also involves interaction of Yap7 with the Hap2/3/5 complex through a conserved Hap4-like-bZIP domain, but this interaction has been lost in C. glabrata. The evolutionary origin of this regulation was investigated by functional analyses of Yap7 and of its paralogue Yap5 in different yeast species. These analyses indicated that the negative regulation of YHB1 by Yap7 arose by neofunctionalization after the whole genome duplication which led to the C. glabrata and S. cerevisiae extant species. This work describes a new aspect of the regulation of fungal nitric oxidase and provides detailed insights into its functioning and evolution. This article is protected by copyright. All rights reserved.
    Molecular Microbiology 03/2015; DOI:10.1111/mmi.12983 · 5.03 Impact Factor