Expression of a subcloned alpha‐amylase gene under the control of a Xanthomonas campestris promoter

Using the promoter probe pKK232‐8 a 0.6‐kb fragment containing an active promoter sequence from Xanthomonas campestris pv campestris was cloned. Two new plasmids were constructed: (a) pAP2, which contains the amy gene from Bacillus subtilis cloned between the Eco RI and Hin dIII sites in the pMFY40 plasmid, and (b) pAP2X, obtained after introduction of the cloned X. campestris promoter upstream from the amy gene. These plasmids were introduced into amylolytic and non‐amylolytic strains of X. campestris pv campestris and pv manihotis, respectively. Quantification of alpha‐amylase specific activity in liquid culture showed that the introduction of a Xanthomonas promoter doubled the expression of amy gene when the host strain was the pathovar campestris but had little effect on the strain from pathovar manihotis. This difference in the promoter activity might indicate that the cloned promoter is specific and could be involved in pathovar differentiation or plant‐pathogen interaction.