A Functional Portrait of Med7 and the Mediator Complex in Candida albicans

Abstract

Mediator is a multi-subunit protein complex that regulates gene expression in eukaryotes by integrating physiological and developmental signals and transmitting them to the general RNA polymerase II machinery. We examined, in the fungal pathogen Candida albicans, a set of conditional alleles of genes encoding Mediator subunits of the head, middle, and tail modules that were found to be essential in the related ascomycete Saccharomyces cerevisiae. Intriguingly, while the Med4, 8, 10, 11, 14, 17, 21 and 22 subunits were essential in both fungi, the structurally highly conserved Med7 subunit was apparently non-essential in C. albicans. While loss of CaMed7 did not lead to loss of viability under normal growth conditions, it dramatically influenced the pathogen's ability to grow in different carbon sources, to form hyphae and biofilms, and to colonize the gastrointestinal tracts of mice. We used epitope tagging and location profiling of the Med7 subunit to examine the distribution of the DNA sites bound by Mediator during growth in either the yeast or the hyphal form, two distinct morphologies characterized by different transcription profiles. We observed a core set of 200 genes bound by Med7 under both conditions; this core set is expanded moderately during yeast growth, but is expanded considerably during hyphal growth, supporting the idea that Mediator binding correlates with changes in transcriptional activity and that this binding is condition specific. Med7 bound not only in the promoter regions of active genes but also within coding regions and at the 3′ ends of genes. By combining genome-wide location profiling, expression analyses and phenotyping, we have identified different Med7p-influenced regulons including genes related to glycolysis and the Filamentous Growth Regulator family. In the absence of Med7, the ribosomal regulon is de-repressed, suggesting Med7 is involved in central aspects of growth control.

Metabolic regulation in model ascomycetes – adjusting similar genomes to different lifestyles

“…There was a clear involvement in Mediator in the switch to filamentous growth, a process linked to virulence and the ability of C. albicans to exploit different niches in the mammalian host [31]. The analysis of the phenotype caused by loss of Med7 [30] also supported a role of Mediator in the development of hyphae. Mutants lacking the MED7 gene were compromised in hyphal development and in biofilm formation, and were defective in colonization of the gastrointestinal tracts of mice.…”

“…8 general, subunits have the same essentiality in both species ( Figure 2). One exception to this rule is the Med7 subunit that forms part of the middle module of the complex, which is essential in S. cerevisiae but can be deleted in C. albicans [30]. The phenotypic consequences of the deletions of several non-essential Mediator subunits have been assessed in C. albicans.…”

“…Analysis of the binding profile of the Med7 protein using ChIP-chip identified a core set of genes that are associated with this Mediator subunit; this core binding set was expanded moderately when cells were growing in the yeast form, but increased more significantly when the cells were growing in the hyphal form. In particular, members of the filamentous growth regulator family and in glycolysis were identified as being enriched for Med7 binding [30]. A separate study that specifically deleted two members of the non-essential kinase module, Med12/Srb8 and Med13/Srb9, showed that these subunits influence general cellular metabolism [32].…”

Budding off: bringing functional genomics toCandida albicans

“…A further development was a library in which one allele was deleted and the second was placed under the control of a tetracycline-repressible promoter [143]. This library, known as the gene replacement and controlled expression (GRACE) collection, includes 2356 genes and has proven useful in assessing conditional lethality [143], functional conservation between S. cerevisiae and C. albicans genes [142], chemical genetic approaches for novel therapeutics [144] and gene function [145]. However, leaky gene expression at some loci can complicate analysis [143] and argues for the use of complementary approaches to define function.…”

Candida albicans morphology: still in focus

“…Clearly, in vivo hyphal formation is mediated by a mix of environmental signals in the host, which includes hypoxia, temperature, CO 2 concentrations, and nutrient conditions, which in turn require a complex network of signaling factors and modules [33,34]. An interesting connection between filament regulation and the general transcription machinery has been found by the discovery that the mediator complex is directly linked to transcriptional regulation of hyphal regulators and hypha-associated genes [35][36][37]. Farnesol is the dominant quorum-sensing molecule and acts via different pathways.…”