Identification of a promoter dependent on NifA and sigma 54 upstream of nifH in Azospirillum lipoferum.

Southern hybridization experiments strongly indicate that the regulatory region of the Azospirillum lipoferum nifH gene is located on a cloned 1.1 kb BamHI-XhoI restriction fragment. By cloning this fragment into a promoter-probe plasmid in Escherichia coli, a promoter was identified oriented towards the nifH gene. Using a set of several bacterial strains and plasmids, both NifA and the alternative sigma factor, sigma 54, from Klebsiella pneumoniae were shown to be required for the induction of the assumed nifH promoter in this particular heterologous system. However, NtrC from K. pneumoniae did not stimulate this promoter. No other promoter activity was detected in the direction opposite to the identified promoter, indicating that the transcription of the adjacent nifJ gene cannot be initiated from the 1.1 kb BamHI-XhoI fragment. Thus, the genes nifH and nifJ in A. lipoferum cannot be oriented divergently, in contrast to the situation in several other nitrogen-fixing bacteria.