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A Thioredoxin of Sinorhizobium meliloti CE52G Is Required for Melanin Production and Symbiotic Nitrogen Fixation

¹Department of Plant Pathology and Microbiology and The Otto Warburg Minerva Center for Agricultural Biotechnology, The Hebrew University of Jerusalem, Faculty of Agricultural, Food and Environmental Quality Sciences, P. O. Box 12, 76100 Rehovot, Israel; ²Departamento de Bioquímica, Instituto Clemente Estable (IIBCE), y Laboratorio de Bioquímica, Sección Biología Molecular y Celular, Facultad de Ciencias, Universidad de la República (UdelaR), Av. Italia 3318, 11600 Montevideo, Uruguay

A miniTn5-induced mutant of a melanin-producing strain of Sinorhizobium meliloti (CE52G) that does not produce melanin was mapped to a gene identified as a probable thioredoxin gene. It was proved that the thiol-reducing activity of the mutant was affected. Addition to the growth medium of substrates that induce the production of melanin (l-tyrosine, guaiacol, orcinol) increased the thioredoxin-like (trxL) mRNA level in the wild-type strain. The mutant strain was affected in the response to paraquat-induced oxidative stress, symbiotic nitrogen fixation, and both laccase and tyrosinase activities. The importance of thioredoxin in melanin production in bacteria, through the regulation of laccase or tyrosinase activities, or both, by the redox state of structural or catalytic SH groups, is discussed.