Feedback regulation of an Agrobacterium catalase gene katA involved in Agrobacterium-plant interaction

Catalases are known to detoxify H2O2, a major component of oxidative stress imposed on a cell. An Agrobacterium tumefaciens catalase encoded by a chro mosomal gene katA has been implicated as an important virulence factor as i t is involved in detoxification of H2O2 released during Agrobacterium-plant interaction. In this paper, we report a feedback regulation pathway that c ontrols the expression of katA in A. tumefaciens cells. We observed that ka tA could be induced by plant tissue sections and by acidic pH on a minimal medium, which resembles the plant environment that the bacteria encounter d uring the course of infection. This represents a new regulatory factor for catalase induction in bacteria. More importantly, a feedback regulation was observed when the katA-gfp expression was studied in different genetic bac kgrounds. We found that introduction of a wild-type katA gene encoding a fu nctional catalase into A. tumefaciens cells could repress the katA-gfp expr ession over 60-fold. The katA gene could be induced by H2O2 and the encoded catalase could detoxify H2O2. In addition, the katA-gfp expression of one bacterial cell could be repressed by other surrounding catalase-proficient bacterial cells. Furthermore, mutation at katA caused a 10-fold increase of the intracellular H2O2 concentration in the bacteria grown on an acidic pH medium. These results suggest that the endogenous H2O2 generated during A. tumefaciens cell growth could serve as the intracellular and intercellular inducer for the katA gene expression and that the acidic pH could pose an oxidative stress on the bacteria. Surprisingly, one mutated KatA protein, e xhibiting no significant catalase activity as a result of the alteration of two important residues at the putative active site, could partially repres s the katA-gfp expression. The feedback regulation of the katA gene by both catalase activity and KatA protein could presumably maintain an appropriat ed level of catalase activity and H2O2 inside A. tumefaciens cells.