Denitrification in Sinorhizobium meliloti

Denitrification is the complete reduction of nitrate or nitrite to N₂, via the intermediates nitric oxide (NO) and nitrous oxide (N₂O), and is coupled to energy conservation and growth under O₂-limiting conditions. In Bradyrhizobium japonicum, this process occurs through the action of the napEDABC, nirK, norCBQD and nosRZDFYLX gene products. DNA sequences showing homology with nap, nirK, nor and nos genes have been found in the genome of the symbiotic plasmid pSymA of Sinorhizobium meliloti strain 1021. Whole-genome transcriptomic analyses have demonstrated that S. meliloti denitrification genes are induced under micro-oxic conditions. Furthermore, S. meliloti has also been shown to possess denitrifying activities in both free-living and symbiotic forms. Despite possessing and expressing the complete set of denitrification genes, S. meliloti is considered a partial denitrifier since it does not grow under anaerobic conditions with nitrate or nitrite as terminal electron acceptors. In the present paper, we show that, under micro-oxic conditions, S. meliloti is able to grow by using nitrate or nitrite as respiratory substrates, which indicates that, in contrast with anaerobic denitrifiers, O₂ is necessary for denitrification by S. meliloti. Current knowledge of the regulation of S. meliloti denitrification genes is also included.