An apparently useless conserved gene in Rhizobium sullae.

In a soil bacterial strain the presence of a specific gene and of the consequent encoded enzyme has generally to be linked to the specific habitat in which the microorganism uses to live. However, there are some examples of genes that are conserved in a bacterial species or strain, but apparently not required by the organism for its surviving, persistence and development in a given environment. A remarkable example of this contradictory circumstance has been observed in some nitrogen fixing, symbiotic bacterial strains of Rhizobium sullae. In their genome these bacteria show a gene, nirK, which encodes for a Cu-containing nitrite reductase known to be part of the reduction chain of nitrate to molecular nitrogen (N2), called denitrification. The gene can be induced under low oxygen concentration giving rise to the conversion of nitrite to nitric oxide (NO). Since the environment where these strains use to live is not characterized by high nitrite concentrations and, mainly, the product of the reaction is an extremely toxic gas, the meaning of the presence of this gene in R. sullae has been extensively investigated. Recently, a multifunction has been proposed for this enzyme on the basis of the ability of these strains to reduce different oxyanions such as selenite. Mutant strains and appropriate fusion proteins were obtained and the expression of the gene was studied under different conditions, including the root nodule elicited by the strains on their natural host legume Hedysarum coronarium.