Carbohydrate binding activities of Bradyrhizobium japonicum: IV. Effect of lactose and flavones on the expression of the lectin, BJ38.

BJ38 is a galactose/lactose-specific lectin (M(r) approximately 38,000) found at one pole of Bradyrhizobium japonicum. It has been implicated in mediating the adhesion of the bacteria to soybean roots, leading to the establishment of a nitrogen-fixing symbiosis. When the ligand lactose is added to cultures of the bacteria for at least 1 h prior to harvesting the cells for BJ38 isolation, the yield of the protein was found to be elevated in a dose-dependent fashion. Half maximal stimulation was observed at approximately 50 microM; the effect was saturated at approximately 1 mM, where a 10-fold higher yield of BJ38 was obtained. Saccharides with a lower affinity for BJ38 than lactose yielded a correspondingly smaller induction effect when compared at a concentration of 1 mM. The higher level of BJ38 induced by lactose is also manifested by an elevated amount of BJ38 detectable at the cell surface and by a higher number of B. japonicum cells adsorbed onto soybean cells. Surprisingly, the induction of BJ38 expression seen with lactose was also observed with certain, but not all, flavonoids that induce the nod genes of the bacteria; genistein mimicked the induction observed with lactose, whereas luteolin failed to stimulate BJ38 production.