The involvement of nitrite reductase of R. sullae in the reduction of different oxyanions

Several strains belonging to the symbiotic, nitrogen fixing species of Rhizobium, Bradyrhizobium and Sinorhizobium have been shown to encode one or more terminal reductases required for denitrification. Some isolates belonging to Rhizobium sullae have been shown to express only a copper-containing nitrite reductase, encoded by nirK, which is closely related to nitrite reductases in true denitrifiers, generating the toxic end product nitric oxide. This enzyme cannot support bacterial growth under anoxic conditions and does not require the presence of a nitrogen oxide for its expression, which depends only upon a decrease in oxygen concentration. This behaviour was previously connected with the low oxygen concentration present within the root nodule, but investigations performed so far on its host legume Hedysarum coronarium have not revealed any significant differences between wild type and nitrite reductase-deficient strains for nodulation efficiency, plant growth and nitrogen fixation. The role of this Cu-containing nitrite reductase was also investigated as a detoxification strategy or as a means to reduce the energy content in the bacterial cell in order to induce the VBNC status, so prolonging cell viability under certain conditions. More recent results suggest that the nitrite reductase of R. sullae strains HCNT1 and A4 can reduce other physiologically important oxyanions, such as selenite, to elemental selenium, and that strains of the same species that lack Nir (e.g. strain CC1335) do not show this property. Moreover, inactivation of nirK in strain HCNT1 resulted in the loss of selenite reduction and the mobilization of nirK into CC1335 produced a phenotype able to reduce selenite. An investigation was also carried out on other rhizobial species known to possess nitrite reductase.