Can nitrite reductase of R. sullae work as selenite reductase?

In some strains of Rhizobium sullae the expression of nirK, a copper containing nitrite reductase, results in the reduction of both nitrite and selenite. While nitrite is converted to nitric oxide (NO) gas, selenite is reduced to elemental selenium. In some R. sulae strains this protein is found to be the unique enzyme of the denitritìcation chain and no energy conservation has ever been detected during nitrite reduction to NO. Therefore, the meaning of this trait in these bacteria has been investigated in order to establish its role. A link with selenite reduction was made evident after (a) the construction of a nirK-minus mutant which completely lost the ability to reduce selenite, (b) the mobilization of a plasmid containing nirK gene into a naturally occurring nitrite reductase detìcient R. sullae strain that increased the resistance to selenite by its reduction to elemental selenium. In the strains containing nirK the reduction of nitrite to NO can be attained only after a preincubation under microaerobic-anaerobic conditions. On the other hand, the reduction of selenite to elemental selenium is occurring aerobically. The presence of this oxyanion in the suspension inhibits nitrite reduction even under anaerobic conditions. In order to purify the protein the expression of nirK in E. coli was tested. Unfortunately these bacteria show a signitìcant selenite reductase activity by themselves. Other nitrite reductase detìcient strains of R sul/ae afe under investigation as appropriate recipient far the expression of nirK.