The bZIP Transcription Factor Rca1p Is a Central Regulator of a Novel CO2 Sensing Pathway in Yeast

Abstract

Like many organisms the fungal pathogen Candida albicans senses changes in the environmental CO2 concentration. This response involves two major proteins: adenylyl cyclase and carbonic anhydrase (CA). Here, we demonstrate that CA expression is tightly controlled by the availability of CO2 and identify the bZIP transcription factor Rca1p as the first CO2 regulator of CA expression in yeast. We show that Rca1p upregulates CA expression during contact with mammalian phagocytes and demonstrate that serine 124 is critical for Rca1p signaling, which occurs independently of adenylyl cyclase. ChIP-chip analysis and the identification of Rca1p orthologs in the model yeast Saccharomyces cerevisiae (Cst6p) point to the broad significance of this novel pathway in fungi. By using advanced microscopy we visualize for the first time the impact of CO2 build-up on gene expression in entire fungal populations with an exceptional level of detail. Our results present the bZIP protein Rca1p as the first fungal regulator of carbonic anhydrase, and reveal the existence of an adenylyl cyclase independent CO2 sensing pathway in yeast. Rca1p appears to regulate cellular metabolism in response to CO2 availability in environments as diverse as the phagosome, yeast communities or liquid culture.

Identification and Functional Characterization of Rca1, a Transcription Factor Involved in both Antifungal Susceptibility and Host Response in Candida albicans

“…Consistent with the study published by Cottier et al (16), we found that carbonic anhydrase (NCE103) was downregulated upon RCA1 deletion. However, several discrepancies were found when comparing our results with those of Cottier et al (16). First, Cottier et al (16) stated that the rca1 mutant was unable to form hyphae, while in our hands, it produced hyphae even under yeast-inducing conditions.…”

“…We focused our investigations on the orf19.6102 mutant, since it exhibited resistance to FLC and CAS and thus was likely to contribute to understanding of crossresistance to drugs with different modes of action. Interestingly, while the present work was ongoing, this TF was identified as the regulator of carbonic anhydrase expression (16) and was consequently named RCA1 (for regulator of carbonic anhydrase). To characterize the function of this TF in addition to its role in CO 2 sensing, and to understand how it modulates antifungal susceptibility, we performed strand-specific RNAseq analyses to identify RCA1-regulated genes by comparing the transcriptomes of RCA1 mutant and revertant strains (raw RNAseq results are available in Table S5 in the supplemental material).…”

“…Cottier et al (16) addressed Rca1 occupancy by chromatin immunoprecipitation with microarray technology (ChIP-onchip) analysis. By merging the expression and ChIP-on-chip data, it is potentially possible to identify direct targets of Rca1.…”

Candida biofilm formation on voice prostheses

“…More recent studies have shown that elevated levels of CO 2 , as found in exhaled breath, can trigger a morphological switch in C. albicans and that this is important for infection and virulence (Cottier et al, 2012;Hall et al, 2010;Klengel et al, 2005). Our preliminary work shows that elevated levels of CO 2 also promote biofilm formation upon VP surfaces (our unpublished results) leading to the possibility that this is a factor that may influence colonization rates in patients fitted with prosthetic devices following larynx removal.…”

Fungal Morphogenesis

“…HCO 3 2 then regulates hyphal morphogenesis through its activation of the adenylyl cyclase Cyr1 and, subsequently, the cAMP-PKA pathway (Klengel et al 2005). HCO 3 2 also signals independently of Cyr1 to regulate levels of carbonic anhydrase (Cottier et al 2012) and promote hyphal development and cell-fate transition (Du et al 2012a). Stabilization of Ume6 by CO 2 is likely mediated through a Cyr1-independent pathway.…”