Only one catalase, katG, is detectable in Rhizobium etli, and is encoded along with the regulator OxyR on a plasmid replicon

Abstract

The plasmid-borneRhizobium etlikatGgene encodes a dual-function catalase-peroxidase (KatG) (EC 1.11.1.7) that is inducible and heat-labile. In contrast to other rhizobia,katGwas shown to be solely responsible for catalase and peroxidase activity inR. etli. AnR. etlimutant that did not express catalase activity exhibited increased sensitivity to hydrogen peroxide (H2O2). Pre-exposure to a sublethal concentration of H2O2allowedR. etlito adapt and survive subsequent exposure to higher concentrations of H2O2. Based on a multiple sequence alignment with other catalase-peroxidases, it was found that the catalytic domains of theR. etliKatG protein had three large insertions, two of which were typical of KatG proteins. Like thekatGgene ofEscherichia coli, theR. etlikatGgene was induced by H2O2and was important in sustaining the exponential growth rate. InR. etli, KatG catalase-peroxidase activity is induced eightfold in minimal medium during stationary phase. It was shown that KatG catalase-peroxidase is not essential for nodulation and nitrogen fixation in symbiosis withPhaseolus vulgaris, although bacteroid proteome analysis indicated an alternative compensatory mechanism for the oxidative protection ofR. etliin symbiosis. Next to, and divergently transcribed from the catalase promoter, an ORF encoding the regulator OxyR was found; this is the first plasmid-encodedoxyRgene described so far. Additionally, thekatGpromoter region contained sequence motifs characteristic of OxyR binding sites, suggesting a possible regulatory mechanism forkatGexpression.