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Mesorhizobium loti MAFF303099 is a rhizobial strain that nodulates Lotus spp. A M. lotiMAFF303099 mutant strain affected in the tatC gene was generated. This strainpresented an altered protein secretion level to the culture supernatant and also a highersensitivity to SDS. Its nodulation phenotype on Lotus showed the induction of small andcolorless nodules, and in a larger number than those induced by the wild-type strain.In addition, these nodules presented defects in the degree of occupation by rhizobia.Two Rieske Fe/S proteins, encoded by the mll2707 and mlr0970 genes, were predictedas potential Tat substrates in M. loti MAFF303099. The transcriptional expression ofmll2707 and mlr0970 genes was analyzed under different oxygen growth conditions.The mll2707 gene was expressed constitutively, while the expression of the mlr0970gene was only detected under anaerobic and microaerophilic in vitro conditions. Bothgenes were down-regulated in the tatC mutant strain. mll2707 and mlr0970 mRNAsfrom the wild-type strain were detected in nodules. Using a translational reporter peptidefusion, we found that the Mll2707 protein was only detectable in the wild-type strain. Onthe other hand, although Mlr0970 protein was detected in wild-type and tatC mutantstrains, its association with the membrane was favored in the wild-type strain. ThetatC and the mll2707 mutant strains were affected in the cytochrome c oxidase activity.These results confirm that Mll2707 is required for cytochrome c-dependent respirationand that Tat functionality is required for the correct activity of Mll2707. The mll2707mutant strain showed a nodulation phenotype similar to the tatC mutant strain, althoughit presented only a slight difference in comparison with wild-type strain in terms of noduleoccupation. No defective phenotype was observed in the nodulation with the mlr0970mutant strain. These results indicate that, of the two Rieske Fe/S proteins coded byM. loti MAFF303099, only Mll2707 expression is required for the induction of effectivenodules, and that the functionality of the Tat system is necessary not only for the correctfunction of this protein, but also for some other protein required in an earlier stage of thenodulation process.