Functional analysis of the fixL/fixJ and fixK genes in Azospirillum brasilense Sp7

Abstract

This study demonstrates for the first time that the two-component FixL/FixJ system, activating expression of nitrogen fixation genes in response to low oxygen concentration in rhizobia, also exists in Azospirillum brasilense Sp7. A heme staining assay showed that A. brasilense FixL contains a heme cofactor like that of rhizobial FixL proteins. Azospirillum brasilense FixL and FixJ could activate expression of Sinorhizobium meliloti nifA in S. meliloti fixL - and fixJ- mutants and in Escherichia coli, and that activation was dependent on oxygen concentration, indicating that A. brasilense FixL and FixJ have the same function as S. meliloti FixL and FixJ. Azospirillum brasilense FixL and FixJ could activate expression of A. brasilense fixK in E. coli. Azospirillum brasilense FixL, FixJ, and FixK positively regulated expression of A. brasilense nifA in A. brasilense strains but not in E. coli. The results show that FixL and FixJ can activate the fixK expression, with FixK subsequently activating nifA expression in A. brasilense. Azospirillum brasilense fixJ was involved in aerotaxis and S. meliloti fixJ could restore aerotaxis of the A. brasilense fixJ- mutant. Mutations in fixL and fixJ did not affect nitrogen fixation. © Springer-Verlag and the University of Milan 2010.