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...

Mitogen-activated protein kinase (MAPK) cascades are frequently used signal transduction mechanisms in eukaryotes. Of the five MAPK cascades in Saccharomyces cerevisiae, the high-osmolarity glycerol response (HOG) pathway functions to sense and respond to hypertonic stress. We utilized a partial loss-of-function mutant in the HOG pathway, pbs2-3, in a high-copy suppressor screen to identify proteins that modulate growth on high-osmolarity media. Three high-copy suppressors of pbs2-3 osmosensitivity were identified: MSG5, CAK1, and TRX1. Msg5p is a dual-specificity phosphatase that was previously demonstrated to dephosphorylate MAPKs in yeast. Deletions of the putative MAPK targets of Msg5p revealed that kss1Δ could suppress the osmosensitivity of pbs2-3. Kss1p is phosphorylated in response to hyperosmotic shock in a pbs2-3 strain, but not in a wild-type strain nor in a pbs2-3 strain overexpressing MSG5. Both TEC1 and FRE::lacZ expressions are activated in strains lacking a functional HOG pathway during osmotic stress in a filamentation/invasion-pathway-dependent manner. Additionally, the cellular projections formed by a pbs2-3 mutant on high osmolarity are absent in strains lacking KSS1 or STE7. These data suggest that the loss of filamentation/invasion pathway repression contributes to the HOG mutant phenotype.