Lectin-Mediated Binding Activities of Bradyrhizobium japonicum

Abstract

Bradyrhizobium japonicum binds to soybean root in a polar fashion (Halverson and Stacey 1986; Ho et al. 1990a). This binding is galactose-specific, suggesting a lectin-mediated binding phenomenon. Further study on the B. japonicum binding property leads to the discovery of four carbohydrate-binding activities: (a) heterotypic binding to soybean root; (b) heterotypic binding to cultured soybean SB-1 cells; (c) homotypic autoagglutination; and (d) adsorption to synthetic beads derivatized with lactose (Ho et al. 1990a). All of these binding activities can be inhibited by galactose or lactose, but not by galactose derivatives, such as N-acetyl-D-galactosamine. These observations suggest a common recognition mechanism involving a lectin residing on the bacterial surface. The purification of a carbohydrate-binding protein from B. japonicum further substantiates this notion (Ho et al. 1990b). This purified lectin, designated BJ38 (M _r~ 38000), showed saccharide specificity that correlated well with that of B. japonicum binding activities. It is proposed that BJ38 may mediate the carbohydrate-binding activities of B. japonicum. Micrographs demonstrating four binding activities of B. japonicum and a model depicting the BJ38-mediated carbohydrate-specific binding of B. japonicum are shown in Figs 1 and 2, respectively. The methods summarized in this chapter are based in part on those reported by Ho et al. (1988, 1990a, b).