Genes controlling early and late functions in symbiosis are located on a megaplasmid in Rhizobium meliloti.

Large plasmids of molecular weight varying from 90 to around 200 x 10(6) have earlier been detected in most Rhizobium meliloti strains using an alkaline denaturation - phenol extraction procedure. With a less destructive method (Eckardt 1978) it was possible additionally to detect one plasmid of molecular weight clearly greater than 300 x 10(6) (= megaplasmid) in all of twenty-seven R. meliloti strains of various geographical origins and nodulation groupings investigated. Four strains (RCR 2011, A145, S26 and CC2013) were found to carry one megaplasmid and no smaller plasmids. Hybridization experiments with Klebsiella pneumoniae and R. meliloti cloned nitrogenase structural genes D and H showed that these genes are located on the megaplasmid and not on the smaller plasmids. All of the ten independent spontaneous non-nodulating derivatives of three strains of R. meliloti were shown to have suffered a deletion in the nifDH region of the megaplasmid. These results indicate that a gene controlling an early step in nodule formation is located in the nifDH region of the megaplasmid. This indicates that the same replicon carries genes controlling early and late functions in symbiosis.