A Novel Denitrification Regulator, Adr, Mediates Denitrification Under Aerobic Conditions in Sinorhizobium meliloti

Abstract

Sinorhizobium meliloti is a soil dwelling bacteria capable of forming a symbiotic relationship with several legume hosts. Once symbiosis is established, S. meliloti fixes atmospheric nitrogen into nitrogenated compounds, thus carrying out an important step in the nitrogen cycle. S. meliloti is also capable of the reverse process, denitrification, the reduction of nitrate and nitrite to nitrogen gas. In this study we have identified a novel regulator of denitrification in S. meliloti, Adr, which affects the expression of the denitrification genes in aerobically grown cultures. Analysis of the Adr sequence reveals a LuxR-like quorum sensing regulator, however, it does not respond to the known quorum sensing signals produced by S. meliloti. Additionally, we show that FixJ, the major regulator of denitrification and microaerobic respiration in S. meliloti, is active under our growth conditions. Comparison of the FixJ microarray to our Adr microarray shows a significant overlap between the two regulons. We also show that while Adr is not necessary for symbiotic nitrogen fixation, a functional copy of this regulator confers a competitive advantage to S. meliloti during host invasion. Our findings suggest that Adr is a new type of denitrification regulator and that it acts at the same regulatory level as FixJ.\n\nImportanceRhizobia contribute to the nitrogen cycle by fixing atmospheric nitrogen to nitrogenated compounds and by denitrification, the reduction nitrate and nitrite to nitrogen gas. Denitrification enhances the survival of Sinorhizobium meliloti in the various environments it may encounter, such as free-living conditions in the rhizosphere, during invasion of the plant host, and after a symbiotic relationship has been established. Oxygen concentration is the typical signal for denitrification gene expression. Recent studies of low oxygen cultures of S. meliloti have outlined the regulation structure for denitrification. In this study, we examine the regulation of denitrification in aerobically grown S. meliloti cultures. Understanding how S. meliloti responds to various oxygen concentrations will result in a more complete picture of denitrification regulation in this agriculturally important organism and the impact of denitrification on the soil microbiome as a whole.