ATYPICAL OXIDATIVE STRESS REGULATION OF A XANTHOMONAS-ORYZAE PV ORYZAE MONOFUNCTIONAL CATALASE

Catalase is an important oxidative stress protective enzyme. We have shown in Xanthomonas oryzae pv. oryzae that catalase has atypical regulation in response to stresses and growth conditions. The superoxide generators paraquat and menadione at subinhibitory concentrations for growth were potent inducers (8.3 and 10.4 fold, respectively) of the catalase of X. oryzae pv. oryzae. In contrast, H2O2 under all conditions tested was a poor inducer of catalase. It gave a maximum induction level of 2.4 fold, while ascorbate produced an intermediate level of induction (4.5 fold). Unexpectedly, methyl methanesulphonate, a DNA methylating agent and mutagen, was a potent inducer of catalase (8 fold). Catalase induction required de novo synthesis of the enzyme and aerobic growth but it was independent of growth phase. Catalase induction by superoxide generators and methyl methanesulphonate was due to increasing synthesis of the 80 000 Da monofunctional catalase. Similar patterns of catalase induction by various agents were observed in all Xanthomonas strains tested, except that paraquat did not induce catalase in the strains with high paraquat resistance. The data illustrate novel aspects of catalase regulation in Xanthomonas strains that differ from that in other microbes. This may have important consequences for plant-microbe interactions.