Roles of HoxX and HoxA in biosynthesis of hydrogenase in Bradyrhizobium japonicum.

In-frame deletion mutagenesis was used to study the roles of two Bradyrhizobium japonicum proteins, HoxX and HoxA, in hydrogenase biosynthesis; based on their sequences, these proteins were previously proposed to be sensor and regulator proteins, respectively, of a two-component regulatory system necessary for hydrogenase transcription. Deletion of the hoxX gene resulted in a strain that expressed only 30 to 40% of wild-type hydrogenase activity. The inactive unprocessed form of the hydrogenase large subunit accumulated in this strain, indicating a role for HoxX in posttranslational processing of the hydrogenase enzyme but not in transcriptional regulation. Strains containing a deletion of the hoxA gene or a double mutation (hoxX and hoxA) did not exhibit any hydrogenase activity under free-living conditions, and extracts from these strains were inactive in gel retardation assays with a 158-bp fragment of the DNA region upstream of the hupSL operon. However, bacteroids from root nodules formed by all three mutant types (hoxX, hoxA, and hoxX hoxA) exhibited hydrogenase activity comparable to that of wild-type bacteroids. Bacteroid extracts from all of these strains, including the wild type, failed to cause a shift of the hydrogenase upstream region used in our assay. It was shown that HoxA is a DNA-binding transcriptional activator of hydrogenase structural gene expression under free-living conditions but not under symbiotic conditions. Although symbiotic hydrogenase expression is still sigma54 dependent, a transcriptional activator other than HoxA functions presumably upstream of the HoxA binding site.