Identification of a Locus Upstream from the Hydrogenase Structural Genes That Is Involved in Hydrogenase Expression in Bradyrhizobium japonicum

A locus involved in the expression of the uptake hydrogenase system of Bradyrhizobium japonicum was identified adjacent to genes encoding the hydrogenase subunits. A cloned fragment of DNA was used to complement to autotrophy a Hup putative regulatory mutant of B. japonicum. The mutant strain lacked hydrogenase activity and synthesized low levels of the large subunit of hydrogenase as determined by Western gels. Tn5-induced mutagenesis located the region within the fragment which was necessary for complementation of the mutant phenotype. The locus identified is adjacent to that encoding the small subunit of hydrogenase; its right border is <0.5 kilobase upstream from the hydrogenase transcriptional start site, and its left border is between 1 and 2.5 kilobases from that start site. However, the locus is outside the region previously shown to contain hup-related genes of B. japonicum. Thus, the localization of this gene describes a previously unidentified hup-related gene on a region of DNA not previously shown to contain hup-specific DNA.