Insight the role of nirK in a non-denitrifying rhizobium.

During the last few years an attempt to determine the role of the gene encoding nitrite reductase, nirK, in some Rhizobium sullae strains was pursued. These strains show the peculiarity of possessing only a nitrite reductase (Nir) without any of the other enzymes needed for the complete reduction of nitrite and nitrate to molecular nitrogen. Once it was verified that the reduction of nitrite to nitric oxide (NO) is not apparently linked to energy conservation and that this Cu-containing enzyme only requires low oxygen for its expression, physiological roles other than nitrite reduction were investigated. More recently an active involvement of Nir of R. sullae in the reduction of other oxyanions was suggested. Particularly, this protein seems to be able to reduce selenite to elemental Se. Further investigation revealed that the selenite reductase does not need low oxygen concentration for its induction, unlike when Nir is serving as a nitrite reductase. Several experiments concerning the introduction of nirK in different rhizobial strains unable to reduce both nitrite and selenite suggested that, although the two activities occur in quite different conditions (aerobic and anaerobic), the enzyme could be the same. Attempts to purify the R. sullae Nir are in progress. This presentation would like to summarize the work made during the last five years of COST 856 research activity.