HOXA IS A TRANSCRIPTIONAL REGULATOR FOR EXPRESSION OF THE HUP STRUCTURAL GENES IN FREE-LIVING BRADYRHIZOBIUM-JAPONICUM

A chromosomally integrated Bradyrhirobium japonicum hoxA mutant is una ble to oxidize hydrogen in free-living conditions. Derepressing condit ions that induce hydrogenase activity in free-living, wild-type B. jap onicum cells cannot induce expression of the hydrogenase structural ge nes in tile hoxA mutant. The DNA-binding capacity of HoxA at the hup p romoter region was studied by means of gel retardation. Both heterotro phically growing cells and cells induced to express hydrogenase activi ty contain a protein that specifically binds to the hup promoter regio n. Crude protein extracts isolated from a B. japonicum hoxA mutant do not contain this binding compound. The HoxA protein was overexpressed in E. coli and isolated in the form of a maltose-binding protein (MBP) -HoxA fusion. The MBP-HoxA hybrid protein specifically bound to a 50 b p region of the hupSL promoter known to be important for regulation of hupSL expression.