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SUMMARY: Plasmid DNA from six strains of Rhizobium leguminosarum was separated by agarose gel electrophoresis and showed a large variation in the number and size of the plasmids. Blotting on to nitrocellulose and hybridization with a probe containing nif (nitrogen fixation) genes from Rhizobium meliloti demonstrated that all strains contain only one plasmid carrying nif genes. The molecular weights of the nif plasmids ranged between approximately 130 x 106 and 550 x 106. Using DNA-DNA hybridization no homologous sequences were detected between the nif plasmid of strain PRE and the other plasmid present in this strain. Hybridizations with RNA from bacteroids showed that only the nif plasmid is strongly expressed in the endosymbiotic state. No selective amplification of plasmid DNA occurs during the transition of free-living bacteria into bacteroids.

Only One of the Large Plasmids in Rhizobium leguminosarum Strain PRE is Strongly Expressed in the Endosymbiotic State