DNA sequence ofRhizobium trifoliinodulation genes reveals a reiterated and potentially regulatory sequence precedingnodABCandnodFE

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Abstract
The Rhizobium trifolii nod genes required for host–specific nodulation of clovers are located on 14 kb of Sym (symbiotic) plasmid DNA. Analysis of the nucleotide sequence of a 3.7 kb portion of this region has revealed open reading frames corresponding to the nodABCDEF genes. A DNA sequencing technique, using primer extension from within Tn 5, has been used to determine the precise locations of Tn 5 mutations within the nod genes and the phenotypes of the corresponding mutants correlate with their mapped locations. The predicted nodA and nodB genes overlap by four nucleotides and the nod F and nodE genes overlap by a single nucleotide, suggesting that translational coupling may ensure the synthesis of equimolar amounts of these gene products. ThenodABC and nodFE genes constitute separate transcriptional units and each is preceded by a conserved 76–bp sequence which may be involved in the regulation of expression of these genes.