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Upon infection of a host, the pathogenic fungus Aspergillus fumigatus is attacked by the reactive oxygen species produced by phagocytic cells. Detoxification of hydrogen peroxide by catalases was proposed as a way to overcome this host response. A. fumigatus produces three active catalases; one is produced by conidia, and two are produced by mycelia. The mycelial catalase Cat1p was studied previously. Here we characterized the two other catalases, their genes, and the phenotypes of gene-disrupted mutants. CatAp, a spore-specific monofunctional catalase, is resistant to heat, metal ions, and detergent. This enzyme is a dimeric protein with 84.5-kDa subunits. The 749-amino-acid polypeptide exhibits high levels of similarity to the Aspergillus nidulans CatA catalase and to bacterial catalase HPII of Escherichia coli. In spite of increased sensitivity to H 2 O 2 , killing of ⌬catA conidia by alveolar macrophages and virulence in animals were similar to the killing of conidia by alveolar macrophages and virulence in animals observed for the wild type. In contrast to the Cat1p and CatAp catalases, the mycelial Cat2p enzyme is a bifunctional catalase-peroxidase and is sensitive to heat, metal ions, and detergent. This enzyme, an 82-kDa monomer, is homologous to catalase-peroxidases of several fungi and bacteria. Surprisingly, mycelium of the double ⌬cat1⌬cat2 mutant with no catalase activity exhibited only slightly increased sensitivity to H 2 O 2 and was as sensitive to killing by polymorphonuclear neutrophils as mycelium of the wild-type strain. However, this mutant exhibited delayed infection in the rat model of aspergillosis compared to infection by the wild-type strain. These results indicate that conidial catalase is not a virulence factor and that mycelial catalases transiently protect the fungus from the host.The opportunistic fungal pathogen Aspergillus fumigatus is responsible for a variety of respiratory diseases in humans, such as allergic bronchopulmonary aspergillosis, aspergilloma, and invasive aspergillosis (10). This fungus is an airborne saprophyte that is inhaled by every human. Alveolar macrophages and polymorphonuclear cells, cellular components of the innate defense of the lung, cooperate to control and eliminate the fungus in the airways. Macrophages eliminate conidia, and protection against the hyphal form is mediated by polymorphonuclear cells (41). Reactive oxygen species (ROS) produced by alveolar macrophages play an essential role in the killing of A. fumigatus conidia (38a). Moreover, in vitro studies of neutrophil function have shown that hydrogen peroxide effectively kills fungal hyphae (12) and that neutrophil-mediated damage is blocked by addition of a commercial catalase (13). Accordingly, catalase, which is a good scavenger of H 2 O 2 , was considered to be a putative virulence factor of A. fumigatus that could counteract the oxidative defense reactions of the host phagocytes (20). No conidial catalase has been identified previously in A. fumigatus. In Aspergillus nidulans, however, a...

Catalases are ubiquitous hydrogen peroxide-detoxifying enzymes that are central to the cellular antioxidant response. Of two catalase activities detected in the fungus Aspergillus nidulans, the catA gene encodes the spore-specific catalase A (CatA). Here we characterize a second catalase gene, identified after probing a genomic library with catA, and demonstrate that it encodes catalase B. This gene, designated catB, predicts a 721-amino-acid polypeptide (CatB) showing 78% identity to an Aspergillus fumigatus catalase and 61% identity to Aspergillus niger CatR. Notably, similar levels of identity are found when comparing CatB to Escherichia coli catalase HPII (43%), A. nidulans CatA (40%), and the predicted peptide of a presumed catA homolog from A. fumigatus (38%). In contrast, the last two peptides share a 79% identity. The catalase B activity was barely detectable in asexual spores (conidia), disappeared after germination, and started to accumulate 10 h after spore inoculation, throughout growth and conidiation. The catB mRNA was absent from conidia, and its accumulation correlated with catalase activity, suggesting that catB expression is regulated at the transcription level. In contrast, the high CatA activity found in spores was lost gradually during germination and growth. In addition to its developmental regulation, CatB was induced by H 2 O 2 , heat shock, paraquat, or uric acid catabolism but not by osmotic stress. This pattern of regulation and the protective role against H 2 O 2 offered by CatA and CatB, at different stages of the A. nidulans life cycle, suggest that catalase gene redundancy performs the function of satisfying catalase demand at the two different stages of metabolic and genetic regulation represented by growing hyphae versus spores. Alternative H 2 O 2 detoxification pathways in A. nidulans were indicated by the fact that catA/catB double mutants were able to grow in substrates whose catabolism generates H 2 O 2 .