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Background  The TolC protein from  Sinorhizobium meliloti  has previously been demonstrated to be required for establishing successful biological nitrogen fixation symbiosis with  Medicago sativa . It is also needed in protein and exopolysaccharide secretion and for protection against osmotic and oxidative stresses. Here, the transcriptional profile of free-living  S. meliloti  1021  tolC  mutant is described as a step toward understanding its role in the physiology of the cell.    Results  Comparison of  tolC  mutant and wild-type strains transcriptomes showed 1177 genes with significantly increased expression while 325 had significantly decreased expression levels. The genes with an increased expression suggest the activation of a cytoplasmic and extracytoplasmic stress responses possibly mediated by the sigma factor RpoH1 and protein homologues of the CpxRA two-component regulatory system of Enterobacteria, respectively. Stress conditions are probably caused by perturbation of the cell envelope. Consistent with gene expression data, biochemical analysis indicates that the  tolC  mutant suffers from oxidative stress. This is illustrated by the elevated enzyme activity levels detected for catalase, superoxide dismutase and glutathione reductase. The observed increase in the expression of genes encoding products involved in central metabolism and transporters for nutrient uptake suggests a higher metabolic rate of the  tolC  mutant. We also demonstrated increased swarming motility in the  tolC  mutant strain. Absence of functional TolC caused decreased expression mainly of genes encoding products involved in nitrogen metabolism and transport.    Conclusion  This work shows how a mutation in the outer membrane protein TolC, common to many bacterial transport systems, affects expression of a large number of genes that act in concert to restore cell homeostasis. This finding further underlines the fundamental role of this protein in  Sinorhizobium meliloti  biology.

Absence of functional TolC protein causes increased stress response gene expression in Sinorhizobium meliloti