Structural and functional characterization of the lexA gene of Xanthomonascampestris pathovar citri

The role of the LexA protein and, specifically, its effect on recA expressi on were analyzed in Xanthomonas campestris pathovar citri (X.c. pv. citri). Overexpression of LexA from X.c. pv. citri, in the plant pathogen, as well as in Escherichia coli, results in increased sensitivity to the DNA-damagi ng agents mitomycin C and ultraviolet radiation, indicating that the recomb inant X.c. pv. citri LexA protein is functional in a different bacterial sp ecies. Immunoblot analysis revealed that the overexpressed LexA protein fun ctioned as a repressor of recA expression in X.c. pv. citri, and that the m itomycin C-induced increase in the abundance of RecA was accompanied by spe cific proteolysis of LexA that required RecA. Although the LexA protein fro m X.c. pv. citri also blocked the expression of recA in E. coli, the E. col i RecA protein was not able to support the autocatalytic cleavage of LexA f rom the plant pathogen. The transcription start site of the X.c. pv. citri lexA gene was identified, and the region upstream of this gene was shown to confer responsiveness to mitomycin C on a luciferase reporter gene constru ct. Electrophoretic mobility-shift assays demonstrated that X.c. pv. citri LexA interacts with the promoter region of X.c. pv. citri lexA, as well as with those of the recA genes of X.c. pv. citri and E. coli. These results i ndicate that LexA functions as a repressor of gene expression in X.c. pv. c itri just as it does in E. coli.