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

Catalase is an important oxidative stress protective enzyme. We have s hown in Xanthomonas oryzae pv. oryzae that catalase has atypical regul ation in response to stresses and growth conditions. The superoxide ge nerators paraquat and menadione at subinhibitory concentrations for gr owth were potent inducers (8.3 and 10.4 fold, respectively) of the cat alase of X. oryzae pv. oryzae. In contrast, H2O2 under all conditions tested was a poor inducer of catalase. It gave a maximum induction lev el of 2.4 fold, while ascorbate produced an intermediate level of indu ction (4.5 fold). Unexpectedly, methyl methanesulphonate, a DNA methyl ating agent and mutagen, was a potent inducer of catalase (8 fold). Ca talase induction required de novo synthesis of the enzyme and aerobic growth but it was independent of growth phase. Catalase induction by s uperoxide generators and methyl methanesulphonate was due to increasin g synthesis of the 80 000 Da monofunctional catalase. Similar patterns of catalase induction by various agents were observed in all Xanthomo nas strains tested, except that paraquat did not induce catalase in th e strains with high paraquat resistance. The data illustrate novel asp ects of catalase regulation in Xanthomonas strains that differ from th at in other microbes. This may have important consequences for plant-m icrobe interactions.