Molecular linkage of the nifl fix and nod gene regions in Rhixobium leguminosarum biovear trifolii

Nucleotide sequence analysis of a 2.5kb region downstream of the nifA gene from Rhizobium leguminosarum biovar trifolii has resulted in linkage, at the DNA sequence level, of the nifEN, nifHDK, fixABCX, nifA gene cluster with the nodEF, nodD, nodABCIJ genes. Four genes have been identified within this intervening region. Immediately 3’to the nifA gene is the nifB gene and the nifB‐linked ferredoxin‐encoding fdxN gene. Downstream of fdxN in R. leguminosarum bv. trifolii and in Rhizobium meliloti, we have identified an open reading frame which has not been described previously and which we propose to designate fixU. Downstream of fixU in R. leguminosarum by. trifolii is a nod gene, nodT, which is contiguous with nodJ (B. Surin et al., manuscript in preparation). As a result of this study, the linkage relationships of 22 symbiotic genes spanning a 24 kb region of the symbiotic plasmid from R. leguminosarum bv. trifoiii are now known.