Nitrate/Nitrite Reducing System in Staphylococcus carnosus: Functional analysis of NirR and NreABC

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Staphylococcus carnosus is traditionally used as a starter culture in the production of raw fermented sausages and dry-cured ham. We have shown that S. carnosus reduces nitrate to ammonia in a two-step mechanism. First, nitrate is reduced to nitrite by a dissimilatory nitrate reductase. Nitrite accumulates in the growth medium and is further reduced by a NADH-dependent nitrite reductase. Nitrite reduction only occurs in the absence of nitrate. Synthesis of both enzymes is induced by anaerobiosis and by the presence of nitrate and/or nitrite. At present, we have identified 26 genes which are involved in nitrate/nitrite reduction. Our aim is to elucidate the underlying regulatory mechanisms which ensure the hierarchical use of the electron acceptors oxygen, nitrate, and nitrite. NirR and NreABC are putative regulators of the nitrate/nitrite reducing system in S. carnosus and we are interested in their mode of action. Especially, we have NreB in view, since it might be involved in sensing and transmitting two different signals: (i) the presence of oxygen, and (ii) the presence of nitrate and/or nitrite. Understanding of the complex regulation of nitrate and nitrite reduction in S. carnosus would be a tool for studying the bioenergetic aspects of the system.