- In Rhizobium meliloti, the operon associated with the nod box n5 comprises nodL, noeA and noeB, three host-range genes specifically required for the nodulation of particular Medicago species. Ardourel, M., Lortet, G., Maillet, F., Roche, P., Truchet, G., Promé, J.C., Rosenberg, C. Mol. Microbiol. (1995)