Novel Regulatory Function for the CCAAT-Binding Factor in Candida albicans

Candida albicans is an opportunistic human pathogen that can sense environmental changes and respond by altering its cell morphology and physiology. A number of environmental factors have been shown to influence this dimorphic transition, including pH, starvation, serum, and amino acids. In this report, we investigate the function of theC. albicans CCAAT-binding factor. InSaccharomyces cerevisiae , this heterooligomeric transcriptional activator stimulates the expression of genes that encode proteins involved in respiration. To examine the function of this transcription factor inC. albicans , we clonedCaHAP5 and generated ahap5Δ/hap5Δ mutant ofC. albicans . Using mobility shift studies, we identified four separate complexes fromC. albicans cell extracts whose DNA-binding activities were abolished in thehap5Δ/hap5Δ mutant, suggesting that they represented sequence-specific CCAAT-binding complexes. We found that theC. albicans hap5Δ homozygote was defective in hyphal development under a variety of conditions, and the mutant displayed a carbon source-dependent “hyperfilamentation” phenotype under certain growth conditions. In addition, the mRNA levels for two enzymes involved in respiration, encoded byCOX5 andCYC1 , were overexpressed in thehap5Δ/hap5Δ mutant when grown in medium containing amino acids as the sole carbon and nitrogen source. Thus, theC. albicans CCAAT-binding factor appeared to function as a repressor of genes encoding mitochondrial electron transport components, in contrast to its activator function inS. cerevisiae . These data provide the first evidence that the CCAAT-binding factor can act as a transcriptional repressor and raise new and interesting questions about how carbon metabolism is regulated in this opportunistic human pathogen.