Telomeric <i>ORFs</i> (<i>TLO</i>s) in <i>Candida</i> spp. Encode Mediator Subunits That Regulate Distinct Virulence Traits

<div><p>The <i>TLO</i> genes are a family of telomere-associated ORFs in the fungal pathogens <i>Candida albicans</i> and <i>C. dubliniensis</i> that encode a subunit of the Mediator complex with homology to Med2. The more virulent pathogen <i>C. albicans</i> has 15 copies of the gene whereas the less pathogenic species <i>C. dubliniensis</i> has only two (<i>CdTLO1</i> and <i>CdTLO2</i>). In this study we used <i>C. dubliniensis</i> as a model to investigate the role of <i>TLO</i> genes in regulating virulence and also to determine whether <i>TLO</i> paralogs have evolved to regulate distinct functions. A <i>C. dubliniensis tlo1</i>Δ/<i>tlo2</i>Δ mutant is unable to form true hyphae, has longer doubling times in galactose broth, is more susceptible to oxidative stress and forms increased levels of biofilm. Transcript profiling of the <i>tlo1</i>Δ/<i>tlo2</i>Δ mutant revealed increased expression of starvation responses in rich medium and retarded expression of hypha-induced transcripts in serum. ChIP studies indicated that Tlo1 binds to many ORFs including genes that exhibit high and low expression levels under the conditions analyzed. The altered expression of these genes in the <i>tlo1</i>Δ/<i>tlo2</i>Δ null mutant indicates roles for Tlo proteins in transcriptional activation and repression. Complementation of the <i>tlo1</i>Δ/<i>tlo2</i>Δ mutant with <i>TLO1</i>, but not <i>TLO2</i>, restored wild-type filamentous growth, whereas only <i>TLO2</i> fully suppressed biofilm growth. Complementation with <i>TLO1</i> also had a greater effect on doubling times in galactose broth. The different abilities of <i>TLO1</i> and <i>TLO2</i> to restore wild-type functions was supported by transcript profiling studies that showed that only <i>TLO1</i> restored expression of hypha-specific genes (<i>UME6, SOD5</i>) and galactose utilisation genes (<i>GAL1</i> and <i>GAL10</i>), whereas <i>TLO2</i> restored repression of starvation-induced gene transcription. Thus, Tlo/Med2 paralogs encoding Mediator subunits regulate different virulence properties in <i>Candida</i> spp. and their expansion may account for the increased adaptability of <i>C. albicans</i> relative to other <i>Candida</i> species.</p>