Identification of a New Inducible Nodulation Gene in Azorhizobium caulinodans. Koen Goethals. Laboratorium voor Genetica, Universiteit Gent, B-9000 Gent, Belgium. Peter Mergaert, Mengsheng Gao, Danny Geelen, Marc Van Montagu, and Marcelle Holsters. Laboratorium voor Genetica, Universiteit Gent, B-9000 Gent, Belgium.. MPMI 5:405-411. Accepted 20 July 1992. Copyright 1992 The American Phytopathological Society.

The narrow host range bacterial strain Azorhizobium caulinodans ORS571 induces the formation of nitrogen-fixing nodules on the root and stem of the tropical legume Sesbania rostrata. Here, a new flavonoid-inducible locus of ORS571 is described, locus 4. The locus was identified and isolated via the occurrence of particular sequences, the γ and δ elements. These elements are reiterated in the ORS571 genome, linked to symbiotic loci. Sequencing of locus 4 showed the presence of an open reading frame (ORF6) that is flanked downstream by a γ element and upstream by a δ element. The γ element is approximately 180 bp in size, and shows homology to the insertion element ISRm3, an insertion sequence belonging to a distinct class of IS elements. The δ element is about 300 bp in size and has homology with repeated sequences found in other Rhizobiaceae. The ORF6 gene product shows a low, but significant homology to the mouse mastocytoma antigen P35B (Szikora et al., EMBO J. 9: 1041-1050, 1990) and to a class of NAD/NADP-binding sugar epimerase/dehydrogenases (Pissowotzki et al., Mol. Gen. Genet. 231: 113-213, 1991). Immediately upstream from ORF6, a nod box-related sequence is present, the arrangement of which is fully consistent with a recently presented model for the nod box structure (Goethals et al., Proc. Natl. Acad. Sci. USA 89: 1646-1650, 1992). Insertional inactivation of ORF6 did not affect the nodulation and fixation performance on S. rostrata. However, on S. formosa roots the nodulation kinetics of such a mutant was clearly affected (about 5 days delay). We propose to call this new symbiotic gene nolK.