Summary: Plasmid-encoded symbiotic determinants from the strain MA1 (817) with uptake hydrogenase activity (Hup) and from the Hup strain MC1 (18a) were mobilized by recombination with the self-transmissible plasmid pVW5JI. The symbiotic determinants were transferred by conjugation from strain MA1 to strain MC1 and to a derivative of MC1 without the symbiotic plasmid, and , thus constructing four types of transconjugants. The determinants for a total recycling hydrogenase in strain MA1 were found to be encoded on the symbiotic plasmid.

Strain MC1 fixed 60% more N in pea root nodules, determined as mg nitrogen per plant, than strain MA1. This difference was not increased in the MC1 derivative that obtained hydrogenase activity. Plants inoculated with a derivative of strain MA1, however, where the symbiotic plasmid was replaced by that of strain MC1 had a high percentage nitrogen content. It was concluded that the symbiotic plasmid and the genetic background were more important for plant nitrogen accumulation than uptake hydrogenase.


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