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1 Programa de Ingeniería Metabólica, Centro de Investigación sobre Fijación de Nitrógeno, Universidad Nacional Autónoma de México, Apdo. Postal 565-A, Cuernavaca, Morelos, CP62210, Mexico
2 Programa de Genética Molecular de Plásmidos Bacterianos, Centro de Investigación sobre Fijación de Nitrógeno, Universidad Nacional Autónoma de México, Apdo. Postal 565-A, Cuernavaca, Morelos, CP62210, Mexico
Correspondence
Sergio Encarnación
encarnac{at}cifn.unam.mx
The plasmid-borne Rhizobium etli katG gene encodes a dual-function catalase-peroxidase (KatG) (EC 1.11.1.7) that is inducible and heat-labile. In contrast to other rhizobia, katG was shown to be solely responsible for catalase and peroxidase activity in R. etli. An R. etli mutant that did not express catalase activity exhibited increased sensitivity to hydrogen peroxide (H2O2). Pre-exposure to a sublethal concentration of H2O2 allowed R. etli to 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 the R. etli KatG protein had three large insertions, two of which were typical of KatG proteins. Like the katG gene of Escherichia coli, the R. etli katG gene was induced by H2O2 and was important in sustaining the exponential growth rate. In R. 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 with Phaseolus vulgaris, although bacteroid proteome analysis indicated an alternative compensatory mechanism for the oxidative protection of R. etli in symbiosis. Next to, and divergently transcribed from the catalase promoter, an ORF encoding the regulator OxyR was found; this is the first plasmid-encoded oxyR gene described so far. Additionally, the katG promoter region contained sequence motifs characteristic of OxyR binding sites, suggesting a possible regulatory mechanism for katG expression.
The katG and oxyR sequences discussed in this study have been deposited in GenBank under accession number AF486647.
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