The role of integraase on the evolution of nod genes of Rhizobium

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The integrase/tRNA gene unit, which mediates site specific recombination and is often found on the genomes of lysogenic phages, was possible to promote the rearrangement or horizontal transfer of nod genes of rhizobia. Recently, the integrase/tRNA gene unit was found on the transferable large DNA fragment including nod genes and was named symbiotic island of Rhizobium. The diversity of rhizobia from tropical legumes may be an evidence of the frequent rearrangement or horizontal transfer of nod genes. The PCR-primer sets for detection of the integrase/tRNA gene unit were designed based on the DNA sequences of integrases of lysogenic phages and tRNAs. However, the integrase/tRNA gene unit could not be found on the genomes of the topical rhizobium stains isolated from soy bean nodules cultivated in Thailand by PCR.The symbiotic plasmid (pSym) of the clover-Rhizobium strain 4S was transferred to Agrobacterium stain A136 by conjugation. Some of the transconjugant Agrobacterium stains were able to nodulate not only on the normal host, white clover, but also on alfalfa, hairy vetch, and siratro. A characteristic bacteroid differentiation was observed in clover and alfalfa nodules induced by the tansconjugants, although nitrogen-fixing ability was not found. Pulsed field gel electrophoresis revealed that a large fragment including nod genes on the pSym could be translocated on the Agrobacterium genome, however, it is not clear whether the translocation was mediated by the integrase/tRNA gene unit on the pSym or not.