Regulation of Nod Gene Expression: The Role of Nod D Protein

Abstract

Bacteria of the genus Rhizobium are able to establish a symbiosis with leguminous plants resulting in the formation of root nodules in which atmospheric nitrogen is fixed. The nodulation of each bacterial species is restricted to its specific group of host plants (cross-inoculation group). Several nodulation genes, which are involved in determining the host-range, have been identified. In contrast to these host specific nod genes, the five nod genes ABCIJ, which constitute one operon, are functionally interchangeable, i.e. common, between Rhizobium species. Both these common and host specific nod genes are present on large Sym(biose) plasmids in the fast growing Rhizobium species and are regulated at the transcriptional level as one regulon. For the activation of the transcription of this regulon three factors are required (i) a nod box, (ii) an activating flavonoid factor secreted by the roots of leguminous plants and (iii) the nodD gene product.