The transcriptional start site of the Bradyrhizobium japonicum fixBC operon was identified by nuclease S1 mapping. It was located approximately 700 base pairs upstream of fixB and was preceded by a promoter sequence that showed strong homology to the B. japonicum fixA promoter and thus to the general nif consensus promoter sequence. Further transcript mapping experiments revealed that fixA and fixBC transcription in B. japonicum strictly depended on the presence of the regulatory gene nifA and on low oxygen partial pressure. Consistent with these data, chromosomally integrated fixA- and fixB-lacZ fusions expressed beta-galactosidase activity only in the wild type but not in a nifA mutant and only under microaerobic but not aerobic growth conditions. The presence of nifA accounted for a 19-fold and 44-fold activation of the fixA and fixB promoters, respectively. These results show that the fixA and fixBC genes are regulated in a way similar to that of the nitrogenase genes nifH and nifDK. A very peculiar finding was that the fixA and fixB promoters, when they were located on plasmids, could hardly be activated by the NifA protein, irrespective of whether this was tested in Escherichia coli or B. japonicum backgrounds. This is in clear contrast to the situation with nifH and nifD promoters.