Abstract: Recombinant DNA clones according to the invention contain nodABCDIJL genes of B. japonicum. These clones can be used to practice a method wherein a nod- strain of Rhizobium or Bradyrhizobium bacteria lacking one or more of these genes is converted to a nod+ strain by transfering one or more of the missing genes to the strain. According to a further aspect of the invention, nucleotide sequences have been identified which control the expression of nod LABC genes of Bradyrhizobium japonicum in the presence of flavones produced by the plant roots. By linking such sequences with a gene having a desired characteristic, such as a gene encoding for production of a plant parasite toxin, bacterial can be created which selectively express the characteristic only in the presence of the flavone which triggers expression of the gene.