ISOLATION AND CHARACTERIZATION OF THE RHIZOBIUM-TROPICI-NOD FACTOR SULFATION GENES

Rhizobium tropici produces a mixture of sulfated and non-sulfated Nod factors, The genes responsible for the sulfation process in R. tropici strain CFN299 were cloned and sequenced. These genes are homologous t o the nodP, nodQ, and nodH genes from R. meliloti. The identity among the two species is 75% for nodP, 74% for nodQ, and 69% for nodH, NodH resembles sulfotransferases in general and NodQ has the characteristic purine-binding motifs and the PAPS (3'-phosphoadenosine 5'-phosphosul fate) motif. Mutants of NodP and NodH were obtained by site-directed m utagenesis, They are no longer able to synthesize the sulfated Nod fac tor, as was demonstrated in high-pressure liquid chromatography and th in-layer chromatography assays, The NodP(-) mutant had a decreased nod ulation capacity in Phaseolus vulgaris Negro Xamapa bean plants. In co ntrast, NodH(-) and NodP(-) mutants acquired an increased capacity to nodulate the high-nitrogen-fixing bean cultivars N-8-116 and BAT-477. Nodulation was restored to normal levels when the mutants were complem ented with a 16-kb clone carrying the wild-type genes, The role of the sulfate on Nod factors in R. tropici was dependent on the bean cultiv ar and the conditions assayed.