A study on the molecular mechanism of resistance to amicarthiazol in Xanthomonas campestris pv. citri

Three amicarthiazol-resistant mutants (Xuv10, Xuv20 and Xuv40) were obtained by UV induction and used in this study. Minimal inhibition concentrations (MICs) of amicarthiazol against the growth of mutants and wild-type isolate were 400 and 100 microg ml(-1) respectively. Inhibition by amicarthiazol of succinate dehydrogenase (SDH) activities of Xanthomonas campestris pv. citri (Hasse) Dye wild-type isolate (Xcc) and three resistant mutants derived from this isolate were assayed using triphenyltetrazolium chloride (TTC). The SDH activities of these mutants were significantly lower than that of Xcc. The complete nucleotide sequences of four subunits (SdhA, SdhB, SdhC and SdhD) of succinate-ubiquinone oxidoreductase (SQR) were cloned by polymerase chain reaction (PCR) amplification. An amino acid mutation (His229--> Leu229) in sdhB was found to confer resistance of X. campestris pv. citri to amicarthiazol. It is suggested that this mutation alters the SDH complex in some way that prevents binding of amicarthiazol.