CHARACTERIZATION OF AN OUTER-MEMBRANE PREPARATION OF PSEUDOMONAS-SYRINGAE PV MORS-PRUNORUM AND ITS BIOLOGICAL-ACTIVITIES IN PLANTA

Outer membrane extracted with EDTA from Pseudomonas syringae pv. mors- prunorum strain C28 formed a band of density 1.21-1.24 g cm(-3) on iso pycnic sucrose gradients, and contained 30% lipopolysaccharide (LPS), 8% phospholipid, and 16% protein. At a minimum concentration of 28 mu g LPS ml(-1), the outer membrane extract produced silvering in cherry and bean leaves, and prevented hypersensitive necrosis in tobacco in r esponse to viable cells of strain C28. The preparation also prevented the development of disease in cherry. Ultracentrifugation of the extra ct yielded a heterogeneous pellet composed of a firm gel, S-20,w(0) = 39 S, overlaid by a more liquid fraction, S-20,w(0) = 6 S. The biologi cal activity of the 6 S pellet was similar to that of uncentrifuged ma terial, but the 39 S component was active at 1.3 mu g LPS ml(-1). The necrosis-preventative, but not the silvering activity of the uncentrif uged extract was potentiated by treatment with a mixture of Ca2+ and M g2+, and was detectable at 3 mu g LPS ml(-1). Biological activity was not attributable to any specific major protein, but correlated with th e sedimentability of the outer membrane, and was therefore probably de termined by the aggregation state of the LPS moiety. Active outer memb rane was also recovered from culture supernatants. It is suggested tha t phytopathogenic bacteria may release outer membrane materials in ord er to damage host tissue. (C) 1995 Academic Press Limited