Spectroscopic and kinetic studies of Nor1, a cytochrome P450 nitric oxide reductase from the fungal pathogen Histoplasma capsulatum.

The fungal respiratory pathogen Histoplasma capsulatum evades the innate immune response and colonizes macrophages during infection. Although macrophage production of the antimicrobial effector nitric oxide (NO) restricts H. capsulatum growth, the pathogen is able to establish a persistent infection. H. capsulatum contains a P450 nitric oxide reductase homologue (NOR1) that may be important for detoxifying NO during infection. To characterize the activity of this putative P450 enzyme, a 404 amino acid fragment of Nor1p was expressed in Escherichia coli and purified to homogeneity. Spectral characterization of Nor1p indicated that it was similar to other fungal P450 nitric oxide reductases. Nor1p catalyzed the reduction of NO to N2O using NADH as the direct reductant. The K(M) for NO was determined to be 20 microM and the k(cat) to be 5000 min(-1). Together, these results provide evidence for a protective role of a P450 nitric oxide reductase against macrophage-derived NO.

Sre1, an iron-modulated GATA DNA-binding protein of iron-uptake genes in the fungal pathogen Histoplasma capsulatum.

The pathogenic fungus Histoplasma capsulatum requires iron for its survival during macrophage infection. Because iron is toxic at high levels, iron acquisition in pathogenic organisms, including H. capsulatum, is a highly regulated process. In response to excess iron, H. capsulatum represses transcription of genes involved in iron uptake. We report here that SRE1, a gene encoding a GATA-type protein, bound to promoter sequences of genes involved in siderophore biosynthesis. Sre1 had sequence similarity to the fungal negative regulators of siderophore biosynthesis. Expression of SRE1 was reduced under iron-starving conditions, underscoring its role as a negative regulator of genes involved in iron uptake. Sre1p specifically bound DNA containing the 5'-(G/A)ATC(T/A)GATAA-3' sequence, and that binding was both iron- and zinc-dependent. Metal analysis indicated that a substoichiometric amount of iron, predominately Fe (3+), was bound to the purified protein. About 0.5-1 equiv of Fe (3+) per monomer was necessary for full DNA-binding activity. Mutations in the conserved cysteine residues in the cysteine-rich region led to a decrease in bound iron. The loss of iron led to a approximately 2.5-fold decrease in DNA-binding affinity, indicating that iron was directly involved in SRE1 regulation of iron-uptake genes.

Shewanella oneidensis MR-1 H-NOX regulation of a histidine kinase by nitric oxide.

Nitric oxide (NO) signaling in animals controls processes such as smooth muscle relaxation and neurotransmission by activation of soluble guanylate cyclase (sGC). Prokaryotic homologues of the sGC heme domain, called H-NOX domains, have been identified and are generally found in a predicted operon in conjunction with a histidine kinase. Here, we show that an H-NOX protein (SO2144) from Shewanella oneidensis directly interacts with the sensor histidine kinase (SO2145), binds NO in a 5-coordinate complex similar to mammalian sGC, and in that form inhibits the activity of a histidine kinase (SO2145). We also describe the first account of NO formation by S. oneidensis under anaerobic growth conditions derived from nitrate and nitrite. These observations suggest that the S. oneidensis H-NOX and histidine kinase pair function as part of a novel two-component signaling pathway that is responsive to NO formation from higher nitrogen oxides used as electron acceptors when oxygen is low and thereby functioning as an environmental sensor.