The two overlapping Azospirillum brasilense upstream activator sequences have differential effects on nifH promoter activity.

The Azospirillum brasilense nifH promoter is positively controlled by the NifA protein bound to the upstream activator sequences (UASs). Two overlapping UASs located at -191 and -182 were identified with the consensus TGT-N10-ACA motif. The role of the two UASs of Azospirillum brasilense nifH promoter was examined by introducing base substitutions in the NifA binding sites. Both the promoter down phenotype of a mutation in UAS2 and increased activation when UAS1 was mutated reveal that the integrity of the UAS2 is required for the efficient activation of nifH promoter. This atypical NifA-binding site may represent a region interacting with two NifA dimers.