SULFATION OF NOD FACTORS VIA NODHPQ IS NODD INDEPENDENT IN RHIZOBIUM-TROPICI CIAT899

A cosmid from the Rhizobium tropici CIAT899 symbiotic plasmid, contain ing most of the nodulation genes described in this strain, has been is olated. Although this cosmid does not carry a nodD gene, it confers ab ility to heterologous Rhizobium spp, to nodulate R. tropici hosts (Pha seolus vulgaris, Macroptilium atropurpureum, and Leucaena leucocephala ). The observed phenotype is due to constitutive expression of the nod ABCSUIJ operon, which has lost its regulatory region and is expressed from a promoter present in the cloning vector. Thin-layer chromatograp hy (TLC) analysis of the Nod factors produced by this construction sho ws that it is still capable of synthesizing sulfated compounds, sugges ting that the nodHPQ genes are organized as an operon that is transcri bed in a nodD-independent manner and is not regulated by flavonoids.