How nirK of R. sullae HCNT1 is involved in selenite reduction

A nitrogen fixing symbiotic strain of Rhizobium sullae, HCNT1, contains a nitrite reductase producing nitric oxide, but not a complementary nitric oxide reductase and the other enzymes required for a complete denitrification pathway. The function of this enzyme is still unknown. Recently this strain was found to be able to grow on selenite concentrations as high as 50 mM and during growth selenite was reduced to the less toxic elemental selenium. A mutant of HCNT1 lacking nitrite reductase showed no evidence of selenite reduction, grew poorly in the presence of 5 mM selenite and was unable to grow in the presence of 25 or 50 mM selenite. Other strains isolated from the same site where HCNT1 was originally collected, showed a similar behaviour of HCNT1. A naturally occurring nitrite reductase deficient R. sullae strain, CC1335, isolated from a quite different site, was found unable to grow in the presence of selenite. Mobilization of a plasmid containing the HCNT1 gene encoding nitrite reductase into CC1335 increased its resistance to this oxyanion. In the presence of nitrite, increasing concentrations of selenite into the buffer containing induced cells of HCNT1 and the nirK+ mutant strain of CC1335 result in a gradual reduction of nitric oxide production. These data suggest that the nitrite reductase of R. sullae provides resistance to selenite indicating a possible explanation for the radically truncated denitrification chain found uniquely in this bacterium.