Abstract: Bacteria of the genus Rhizobium nodulate legumes and are responsible for nitrogen fixation. The energetics of this process are improved if hydrogen uptake ability (Hup) is imparted to the bacteria to "recycle" some of the hydrogen lost during the nitrogen fixation. Such Hup is observed in a naturally occurring strain of Rhizobium leguminosarum but it was not known how to transfer it to other strains. The present invention provides recombinant plasmids pIJ1008 and pIJ1007 which enable it to be transferred. These plasmids are formed from a transmissible plasmid, synthesized to include a drug-resistance marker, and a plasmid of the naturally occurring strain. Cultures containing the plasmids have been deposited in a culture collection and can be used, after appropriate further crosses as described, to impart Hup to other strains of Rhizobium.