Morphological transitions and metabolic regulation are critical for the human fungal pathogen Candida albicans to adapt to the changing host environment. In this study, we generated a library of central metabolic pathway mutants in the tricarboxylic acid (TCA) cycle, and investigated the functional consequences of these gene deletions on C. albicans biology. Inactivation of the TCA cycle impairs the ability of C. albicans to utilize non-fermentable carbon sources and dramatically attenuates cell growth rates under several culture conditions. By integrating the Ras1-cAMP signaling pathway and the heat shock factor-type transcription regulator Sfl2, we found that the TCA cycle plays fundamental roles in the regulation of CO 2 sensing and hyphal development. The TCA cycle and cAMP signaling pathways coordinately regulate hyphal growth through the molecular linkers ATP and CO 2. Inactivation of the TCA cycle leads to lowered intracellular ATP and cAMP levels and thus affects the activation of the Ras1-regulated cAMP signaling pathway. In turn, the Ras1-cAMP signaling pathway controls the TCA cycle through both Efg1- and Sfl2-mediated transcriptional regulation in response to elevated CO 2 levels. The protein kinase A (PKA) catalytic subunit Tpk1, but not Tpk2, may play a major role in this regulation. Sfl2 specifically binds to several TCA cycle and hypha-associated genes under high CO 2 conditions. Global transcriptional profiling experiments indicate that Sfl2 is indeed required for the gene expression changes occurring in response to these elevated CO 2 levels. Our study reveals the regulatory role of the TCA cycle in CO 2 sensing and hyphal development and establishes a novel link between the TCA cycle and Ras1-cAMP signaling pathways.
Energy metabolism through the TCA cycle and mitochondrial electron transport are critical for the human fungal pathogen Candida albicans to survive and propagate in the host. This is, in part, due to the fact that C. albicans is a Crabtree-negative species, and thus exclusively uses respiration when oxygen is available. Here, we investigate the roles of the TCA cycle in hyphal development and CO 2 sensing in C. albicans. Through the use of ATP and the cellular signaling molecule CO 2, the TCA cycle integrates with the Ras1-cAMP signaling pathway, which is a central regulator of hyphal growth, to govern basic cellular biological processes. Together with Efg1, a downstream transcription factor of the cAMP signaling pathway, the heat shock factor-type transcription regulator Sfl2 controls CO 2-induced hyphal growth in C. albicans. Deletion of SFL2 results in the loss of global transcriptional responses under elevated CO 2 levels. Our study indicates that the TCA cycle not only occupies the central position of cellular metabolism but also regulates other biological processes such as CO 2 sensing and hyphal development through integration with the Ras1-cAMP signaling pathway in C. albicans.