Isolation and expression of Rhizobium japonicum cloned DNA encoding an early soybean nodulation function

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Abstract
A 1st visible step in the nodulation of legumes by Rhizobium spp. is the deformation and curling of root hairs. DNA sequences encoding this function were identified and cloned from 2 strains of R. japonicum (USDA 122 and USDA 110) with a weakly homologous probe from R. meliloti. Root hair curling encoded by the cloned DNA fragments was examined on soybeans (G. soja) after conjugative transfer of these sequences in broad-host-range vectors to various bacterial genera. Pseudomonas putida gave unambiguous expression of the root hair curling genes. This enabled us to identify the 8.7-kilobase (kb) EcoRI fragments encoding root hair curling from each strain. The phenotypes encoded by the plasmids pBS1 (derived from strain USDA 122) and pBS2 (derived from strain USDA 110) are distinct and represent a phenotype characteristic of their parent R. japonicum strains. Subclones of pBS1 and pBS2 were generated in single and multicopy vectors, and their expression was analyzed in P. putida. Evidently, a 4.2-kb internal SalI fragment of pBS1 and a 3.5-kb SstI-EcoRI fragment of pBS2 are sufficient to confer root hair curling on soybeans.