IDENTIFICATION OF A POTENTIAL TRANSCRIPTIONAL REGULATOR OF HYDROGENASE ACTIVITY IN FREE-LIVING BRADYRHIZOBIUM-JAPONICUM STRAINS

In Bradyrhizobium japonicum, Tn5 insertions in a particular chromosoma l DNA fragment result in a Hup- phenotype in free-living conditions wi thout affecting hydrogenase (Hup) activity in the symbiotic state. By determination of the nucleotide sequence of this region, we were able to identify the nature of the inactivated genes. The fragment is locat ed 9 kb downstream of the hydrogenase structural genes and contains on e incomplete and three complete open reading frames. They are designat ed hypD', hypE, hoxX and hoxA respectively, since the deduced amino ac id sequences display very strong homology with genes involved in the r egulation of hydrogenase activity in Escherichia coli, Rhodobacter cap sulatus, Azotobacter vinelandii (hypD' and hypE) and Alcaligenes eutro phus (hoxX and hoxA). This is the first report on transcriptional acti vators of the hup genes in B. japonicum. Implications of these finding s with respect to regulation of hydrogenase synthesis by hydrogen, oxy gen and nickel in free-living B. japonicum are discussed.