The yeast Candida albicans is an opportunistic pathogen that threatens patients with compromised immune systems. Immune cell defenses against C. albicans are complex but typically involve the production of reactive oxygen species and nitrogen radicals such as nitric oxide (NO) that damage the yeast or inhibit its growth. Whether Candida defends itself against NO and the molecules responsible for this defense have yet to be determined. The defense against NO in various bacteria and the yeast Saccharomyces cerevisiae involves an NO-scavenging flavohemoglobin. The C. albicans genome contains three genes encoding flavohemoglobinrelated proteins, CaYHB1, CaYHB4, and CaYHB5. To assess their roles in NO metabolism, we constructed strains lacking each of these genes and demonstrated that just one, CaYHB1, is responsible for NO consumption and detoxification. In C. albicans, NO metabolic activity and CaYHB1 mRNA levels are rapidly induced by NO and NO-generating agents. Loss of CaYHB1 increases the sensitivity of C. albicans to NO-mediated growth inhibition. In mice, infections with Candida strains lacking CaYHB1 still resulted in lethality, but virulence was decreased compared to that in wild-type strains. Thus, C. albicans possesses a rapid, specific, and highly inducible NO defense mechanism involving one of three putative flavohemoglobin genes.The dimorphic fungus Candida albicans causes infections in immunocompromised hosts and is particularly problematic for AIDS and cancer patients. In healthy individuals, phagocytic immune cells such as macrophages (17), monocytes (37, 45), and neutrophils (45) defend against Candida infections by producing several growth inhibitors and cytotoxic compounds, including microbicidal enzymes (41) and reactive oxygen and nitrogen species (50). One potentially powerful weapon against C. albicans is nitric oxide (NO). Macrophages produce high concentrations of this free radical via the action of an inducible NO synthase (36), inhibition of which strongly decreases the candidacidal activity of macrophages (4,15,43). Despite the increasing understanding of host immune defenses mounted against this opportunistic pathogen, the means by which C. albicans resists NO or other microbicidal agents is not well understood.Microbes protect themselves against NO toxicity by using enzymes that convert NO to less toxic molecules. Flavohemoglobin, an NO dioxygenase (NOD) that converts NO to nitrate (26,29,55), is found in bacteria and yeasts (7,58). This enzyme is encoded by a single gene in several different organisms: for example, by hmp in Escherichia coli (49) and by ScYHB1 in Saccharomyces cerevisiae (57). Flavohemoglobin is necessary for virulence of a plant pathogen, the bacterium Erwinia chrysanthemi (18). hmp-negative bacteria are more easily inhibited by 21,26,38), and expression of hmp is strongly induced by NO (8,42). Hmp induction by NO is mediated by a derepression mechanism in which NO inactivates a metal-binding transcription factor, Fnr (10, 42) or Fur (8, 11). In the yeast S. cerevisi...

This work has identified regulatory elements in the major fungal pathogen Candida albicans that enable response to nitrosative stress. Nitric oxide (NO) is generated by macrophages of the host immune system and commensal bacteria, and the ability to resist its toxicity is one adaptation that promotes survival of C. albicans inside the human body. Exposing C. albicans to NO induces upregulation of the flavohemoglobin Yhb1p. This protein confers protection by enzymatically converting NO to harmless nitrate, but it is unknown how C. albicans is able to detect NO in its environment and thus initiate this defense only as needed. We analyzed this problem by incrementally mutating the YHB1 regulatory region to identify a nitric oxide-responsive element (NORE) that is required for NO sensitivity. Five transcription factor candidates of the Zn(II)2-Cys6 family were then isolated from crude whole-cell extracts by using magnetic beads coated with this DNA element. Of the five, only deletion of the CTA4 gene prevented induction of YHB1 transcription during nitrosative stress and caused growth sensitivity to the NO donor dipropylenetriamine NONOate; Cta4p associates in vivo with NORE DNA from the YHB1 regulatory region. Deletion of CTA4 caused a small but significant decrease in virulence. A CTA4-dependent putative sulfite transporter encoded by SSU1 is also implicated in NO response, but C. albicans ssu1 mutants were not sensitive to NO, in contrast to findings in Saccharomyces cerevisiae. Cta4p is the first protein found to be necessary for initiating NO response in C. albicans.