Novel and Complex Chromosomal Arrangement of Rhizobium /otiNodulation Genes. D. Barry Scott. Molecular Genetics Unit, Department of Microbiology and Genetics, Massey University, Palmerston North, New Zealand. Carolyn A. Young (1), Julie M. Collins-Emerson (1), Eric A.Terzaghi (1), Eva S. Rockman(1), Pauline E. Lewis (2), and Clive E. Pankhurst (2). (1) Molecular Genetics Unit, Department of Microbiology and Genetics, Massey University, Palmerston North, New Zealand; (2) Biotechnology Division, Department of Scientific and Industrial Research, Palmerston North, New Zealand. MPMI 9:187-197. Accepted 27 December 1995. Copyright 1996 The American Phytopathological Society.

A mutational and structural analysis of Rhizobium loti nodulation genes in strains NZP2037 and NZP2213 was carried out. Unlike the case with other Rhizobium strains examined to date, nodB was found on an operon separate from nodACIJ. Sequence analysis of the nodACIJ and nodB operon regions confirm that R. loti common nod genes have a gene organization different from that of other Rhizobium spp. At least 4 copies of nodD-like sequences were identified in R. loti. The complete nucleotide sequence of one of these, nodD3, was determined. A new host-specific nod gene, nolL, was identified adjacent to nodD3. NolL shares homology with NodX and other O-acetyl transferases. Mutational analysis of the nod regions of strains NZP2037 and NZP2213 showed that nodD3, nodI, nodJ, and nolL were all essential for R. loti strains to effectively nodulate the extended host Lotus pedunculatus, but were not necessary for effective nodulation of the less restrictive host, Lotus corniculatus. Both nodD3 and nolL were essential for R. loti strains to nodulate Leucaena leucocephala.