Studies on the Regulation of the Assimilatory Nitrate Reductase Operon in Azotobacter vinelandii

Abstract

Azotobacter vinelandii is a free-living diazotroph. This bacterium fixes atmospheric nitrogen in different environments using three genetically distinct nitrogenases. A. vinelandii is also capable of utilizing nitrate and nitrite from the environment. Nitrate is reduced sequentially into nitrite and ammonia. The assimilatory nitrate reductase and nitrite reductase are encoded by the nasAB operon. Previous genetic studies identified a number of factors that influence nasAB expression. However, the molecular mechanisms controlling the expression of nasAB are unclear.The current study was initiated to characterize the region preceding the nasAB operon which was previously implicated in its regulation and to further study the molecular mechanisms of nasAB regulation. The results confirm that nasAB is subject to multiple layers of regulation. The operon is under the control of an NtrC-dependent promoter; nitrate/nitrite induction occurs at the post-transcriptional level via antitermination within the nasAB leader region; and nitrate/nitrite induction is mediated by NasS/NasT, a sensor-antiterminator two-component regulatory system.Together, these data suggest a model for the regulation of the assimilatory nitrate reductase operon in A. vinelandii.