Proteins entering the eukaryotic secretory pathway commonly are glycosylated. Important steps in this posttranslational modification are carried out by mannosyltransferases. In this study, we investigated the putative ␣-1,2-mannosyltransferase AfMnt1 of the human pathogenic mold Aspergillus fumigatus. AfMnt1 belongs to a family of enzymes that comprises nine members in Saccharomyces cerevisiae but only three in A. fumigatus. A ⌬afmnt1 mutant is viable and grows normally at 37°C, but its hyphal cell wall appears to be thinner than that of the parental strain. The lack of AfMnt1 leads to a higher sensitivity to calcofluor white and Congo red but not to sodium dodecyl sulfate. The growth of the mutant is abrogated at 48°C but can be restored by osmotic stabilization. The resulting colonies remain white due to a defect in the formation of conidia. Electron and immunofluorescence microscopy further revealed that the observed growth defect of the mutant at 48°C can be attributed to cell wall instability resulting in leakage at the hyphal tips. Using a red fluorescence fusion protein, we localized AfMnt1 in compact, brefeldin A-sensitive organelles that most likely represent fungal Golgi equivalents. The tumor necrosis factor alpha response of murine macrophages to hyphae was not affected by the lack of the afmnt1 gene, but the corresponding mutant was attenuated in a mouse model of infection. This and the increased sensitivity of the ⌬afmnt1 mutant to azoles, antifungal agents that currently are used to treat Aspergillus infections, suggest that ␣-1,2-mannosyltransferases are interesting targets for novel antifungal drugs.

Soils may comprise tens of thousands to millions of bacterial species. It is still unclear whether this high level of diversity is governed by functional redundancy or by a multitude of ecological niches. In order to address this question, we analyzed the reproducibility of bacterial community composition after different experimental manipulations. Soil lysimeters were planted with four different types of plant communities, and the water content was adjusted. Group-specific phylogenetic fingerprinting by PCR-denaturing gradient gel electrophoresis revealed clear differences in the composition of Alphaproteobacteria, Betaproteobacteria, Bacteroidetes, Chloroflexi, Planctomycetes, and Verrucomicrobia populations in soils without plants compared to that of populations in planted soils, whereas no influence of plant species composition on bacterial diversity could be discerned. These results indicate that the presence of higher plant species affects the species composition of bacterial groups in a reproducible manner and even outside of the rhizosphere. In contrast, the environmental factors tested did not affect the composition of Acidobacteria, Actinobacteria, Archaea, and Firmicutes populations. One-third (52 out of 160) of the sequence types were found to be specifically and reproducibly associated with the absence or presence of plants. Unexpectedly, this was also true for numerous minor constituents of the soil bacterial assemblage. Subsequently, one of the low-abundance phylotypes (beta10) was selected for studying the interdependence under particular experimental conditions and the underlying causes in more detail. This so-far-uncultured phylotype of the Betaproteobacteria species represented up to 0.18% of all bacterial cells in planted lysimeters compared to 0.017% in unplanted systems. A cultured representative of this phylotype exhibited high physiological flexibility and was capable of utilizing major constituents of root exudates. Our results suggest that the bacterial species composition in soil is determined to a significant extent by abiotic and biotic factors, rather than by mere chance, thereby reflecting a multitude of distinct ecological niches.

The mannosyltransferase Och1 is the key enzyme for synthesis of elaborated protein N-glycans in yeast. In filamentous fungi genes implicated in outer chain formation are present, but their function is unclear. In this study we have analyzed the Och1 protein of Aspergillus fumigatus. We provide first evidence that poly-mannosylated N-glycans exist in A. fumigatus and that their synthesis requires AfOch1 activity. This implies that AfOch1 plays a similar role as S. cerevisiae ScOch1 in the initiation of an N-glycan outer chain. A Δafoch1 mutant showed normal growth under standard and various stress conditions including elevated temperature, cell wall and oxidative stress. However, sporulation of this mutant was dramatically reduced in the presence of high calcium concentrations, suggesting that certain proteins engaged in sporulation require N-glycan outer chains to be fully functional. A characteristic feature of AfOch1 and Och1 homologues from other filamentous fungi is a signal peptide that clearly distinguishes them from their yeast counterparts. However, this difference does not appear to have consequences for its localization in the Golgi. Replacing the signal peptide of AfOch1 by a membrane anchor had no impact on its ability to complement the sporulation defect of the Δafoch1 strain. The mutant triggered a normal cytokine response in infected murine macrophages, arguing against a role of outer chains as relevant Aspergillus pathogen associated molecular patterns. Infection experiments provided no evidence for attenuation in virulence; in fact, according to our data the Δafoch1 mutant may even be slightly more virulent than the control strains.

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