Selenite-reducing capacity of the copper-containing nitrite reductase of Rhizobium sullae

Rhizobium sullae strain HCNT1 contains a nitric oxide-producing nitrite reductase of unknown function due to the absence of a complementary nitric oxide reductase. HCNT1 had the ability to grow on selenite concentrations as high as 50 mM, and during growth, selenite was reduced to the less toxic elemental selenium. An HCNT1 mutant lacking nitrite reductase grew poorly in the presence of 5 mM selenite, was unable to grow in the presence of 25 or 50 mM selenite and also showed no evidence of selenite reduction. A naturally occurring nitrite reductase-deficient R. sullae strain, CC1335, also showed little growth on the higher concentrations of selenite. Mobilization of a plasmid containing the HCNT1 gene encoding nitrite reductase into CC1335 increased its resistance to selenite. To confirm that this ability to grow in the presence of high concentrations of selenite correlated with nitrite reductase activity, a new nitrite reductase-containing strain was isolated from the same location where HCNT1 was isolated. This strain was also resistant to high concentrations of selenite. Inactivation of the gene encoding nitrite reductase in this strain increased selenite sensitivity. These data suggest that the nitrite reductase of R. sullae provides resistance to selenite and offers an explanation for the radically truncated denitrification found uniquely in this bacterium.