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

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Abstract
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.Key words: Azospirillum brasilense, nif promoters, upstream activator sequence mutagenesis, NifA binding.