Purification and partial characterization of an agglutinin in citrus leaves against extracellular polysaccharides of Xanthomonas campestris pv. citri

Abstract

An agglutinin against extracellular polysaccharides (EPS) of Xanthomonas campestris pv. citri, the casual agent of citrus canker, was precipitated with 50% saturated ammonium sulphate from an acid (pH 4·0) homogenate of citrus leaf tips. The agglutinin was selectively withdrawn from the crude preparation at pH 4·0 by agglutination with the bacterial EPS, and then separated from the EPS-agglutinin complex on a DEAE cellulose column with a buffer (pH 7·0) containing d-glucosamine, a hapten sugar of the agglutinin. Bio-Gel P-100 gel chromatography of the agglutinin-containing fraction at pH 7·0 and 4·0, yielded a single peak consisting of 96% protein and 4% carbohydrate with estimated molecular masses of 63 000 and 16 000, respectively. The agglutinin ran on SDS-gel electrophoresis as a single band with a molecular mass of 32 000, and on isoelectric focusing gel electrophoresis as a single band with an isoelectric point of pH 4·8.

The purified agglutinin was active with EPS at pH values lower than 6·0, forming a fibrous material. Agglutination was specifically prevented by d-glucosamine. At pH 7·0, the agglutinin associated with EPS via a d-glucosamine-sensitive site without visible precipitation.

A role for the agglutinin in the parasitism of citrus tissue by X. campestris pv. citri is presented.