B.japonicum has two FixLJ-dependent FixK homologs that are activators of the transcription of a group of genes involved in anaerobic processes such as denitrification and possibly nitrogen fixation. FixK may bind DNA at the FNR consensus binding site. Its expression is maximal under reduced oxygen conditions and depends on the two-component regulatory system FixLJ. Possesses 4 cysteines which may bind a metal ion (possibly iron).