PLASMID-BORNE DETERMINANTS OF PIGMENTATION, EXOPOLYSACCHARIDE PRODUCTION, AND VIRULENCE IN XANTHOMONAS-CAMPESTRIS PV MALVACEARUM

Three plasmids of 55.0, 31.2, and 7.4 kb, designated as pXCM18-I, pXCM 18-II, and pXCM18-III, respectively, were identified and isolated from strain HR13B/2 of Xanthomonas campestris is pv. malvacearum. The bact erium was cured of all three plasmids in high frequency (3.1%) with he at (42 degrees C). The cured strains were nonmucoid and light yellow a nd were greatly impaired in exopolysaccharide production and virulence . Transformation of a cured strain using a mixture of purified plasmid DNA from HR13B/2 resulted in colonies carrying all six possible combi nations of the three plasmids. Of the transformants, two morphological ly distinct colony types were observed. The first type of transformant s were mucoid and deep yellow, not distinguished from wild-type coloni es in morphology, and fully complemented for exopolysaccharide product ion. These transformants invariably carried pXCM18-II. The second type of transformants were not distinguished from the untransformed strain in morphology but were found to be transformed after screening for pl asmid content. These transformants carried either pXCM18-I, pXCM18-III , or both, but never pXCM18-II. In pathogenicity tests, each of the th ree plasmids restored virulence significantly, and additively, with pl asmid pXCM18-I playing a predominant role.