Expansion of the TLO gene family enhances the virulence of Candida species.

The TLO genes are a family of subtelomeric ORFs in the fungal pathogens Candida albicans and C. dubliniensis encoding a subunit of the Mediator complex homologous to Med2. The more virulent pathogen C. albicans has 15 copies of the gene whereas the less pathogenic species C. dubliniensis has only two. To investigate if expansion of the TLO repertoire in C. dubliniensis has an effect on phenotype and virulence we expressed three representative C. albicans TLO genes (TLOβ2, TLOγ11 and TLOα12) in a wild type C. dubliniensis background, under the control of either their native or the ACT1 promoter. Expression of TLOβ2 resulted in a hyperfilamentous phenotype, while overexpression of TLOγ11 and TLOα12 resulted in enhanced resistance to oxidative stress. Expression of all three TLO genes from the ACT1 promoter resulted in increased virulence in the Galleria infection model. In order to further investigate if individual TLO genes exhibit differences in function we expressed six representative C. albicans TLO genes in a C. dubliniensis Δtlo1/Δtlo2 double mutant. Differences were observed in the ability of the expressed CaTLOs to complement the various phenotypes of the mutant. All TLO genes with the exception of TLOγ7 could restore filamentation, however only TLOα9, γ11 and α12 could restore chlamydospore formation. Differences in the ability of CaTLO genes to restore growth in the presence of H2O2, calcofluor white, Congo red and at 42°C were observed. Only TLOα3 restored wild-type levels of virulence in the Galleria infection model. These data show that expansion of the TLO gene family in C. dubliniensis results in gain of function and that there is functional diversity amongst members of the gene family. We propose that this expansion of the TLO family contributes to the success of C. albicans as a commensal and opportunistic pathogen.