Two gene clusters of Rhizobium meliloti code for early essential nodulation functions and a third influences nodulation efficiency.

A pLAFR1 cosmid clone (pPP346) carrying the nodulation region of the symbiotic plasmid pRme41b was isolated from a gene library of Rhizobium meliloti 41 by direct complementation of a Nod- deletion mutant of R. meliloti. Agrobacterium tumefaciens and Rhizobium species containing pPP346 were able to form ineffective nodules on alfalfa. The 24-kilobase insert in pPP346 carries both the common nodulation genes and genes involved in host specificity of nodulation. It was shown that these two regions are essential and sufficient to determine the early events in nodulation. A new DNA region influencing the kinetics and efficiency of nodulation was also localized on the symbiotic megaplasmid at the right side of the nif genes.