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A Mutant of Paracoccus denitrificans with Disrupted Genes Coding for Cytochrome c₅₅₀ and Pseudoazurin Establishes These Two Proteins as the In Vivo Electron Donors to Cytochrome cd₁ Nitrite Reductase

Department of Biochemistry, University of Oxford, Oxford OX1 3QU, United Kingdom,¹ Department of Molecular Cell Physiology, Faculty of Biology, Biocentrum Amsterdam, Free University, 1081 HV Amsterdam, The Netherlands²

Received 9 April 2003/ Accepted 8 August 2003

In Paracoccus denitrificans, electrons pass from the membrane-bound cytochrome bc₁ complex to the periplasmic nitrite reductase, cytochrome cd₁. The periplasmic protein cytochrome c₅₅₀ has often been implicated in this electron transfer, but its absence, as a consequence of mutation, has previously been shown to result in almost no attenuation in the ability of the nitrite reductase to function in intact cells. Here, the hypothesis that cytochrome c₅₅₀ and pseudoazurin are alternative electron carriers from the cytochrome bc₁ complex to the nitrite reductase was tested by construction of mutants of P. denitrificans that are deficient in either pseudoazurin or both pseudoazurin and cytochrome c₅₅₀. The latter organism, but not the former (which is almost indistinguishable in this respect from the wild type), grows poorly under anaerobic conditions with nitrate as an added electron acceptor and accumulates nitrite in the medium. Growth under aerobic conditions with either succinate or methanol as the carbon source is not significantly affected in mutants lacking either pseudoazurin or cytochrome c₅₅₀ or both these proteins. We concluded that pseudoazurin and cytochrome c₅₅₀ are the alternative electron mediator proteins between the cytochrome bc₁ complex and the cytochrome cd₁-type nitrite reductase. We also concluded that expression of pseudoazurin is mainly controlled by the transcriptional activator FnrP.

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