ISOLATION OF SINORHIZOBIUM-MELILOTI TN5 MUTANTS WITH ALTERED CYTOCHROME TERMINAL OXIDASE EXPRESSION AND IMPROVED SYMBIOTIC PERFORMANCE

The relationship between whole-cell redox potential, cytochrome compos ition in free-living culture and symbiotic activity of Sinorhizobium m eliloti was studied. Three Tn5-induced mutants with increased cellular redox potential were generated, Stationary cultures of mutants Tb9 an d Tb16 in contrast to the parental strain produced the b-type terminal oxidase that may be similar to the symbiotically essential cytochrome oxidase cbb(3) of Bradyrhizobium japonicum. Increase in the symbiotic effectiveness of all three mutants and in O-2 consumption rate in fre e-living cultures was observed. Mutants Tb1 and Tb16 were also charact erized by an increase in fixNOQP gene expression. Consequently, the mu tations probably affect at least two different steps of rhizobial resp iratory metabolism operating both in free-living cells and endosymbiot ic forms. (C) 1998 Federation of European Microbiological Societies. P ublished by Elsevier Science B.V. All rights reserved.