AUTHOR=Basile Laura A. , Zalguizuri Andrés , Briones Gabriel , Lepek Viviana C. 

TITLE=Two Rieske Fe/S Proteins and TAT System in Mesorhizobium loti MAFF303099: Differential Regulation and Roles on Nodulation

JOURNAL=Frontiers in Plant Science

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2018.01686

DOI=10.3389/fpls.2018.01686

ISSN=1664-462X

ABSTRACT=<p><italic>Mesorhizobium loti</italic> MAFF303099 is a rhizobial strain that nodulates <italic>Lotus</italic> spp. A <italic>M. loti</italic> MAFF303099 mutant strain affected in the <italic>tatC</italic> gene was generated. This strain presented an altered protein secretion level to the culture supernatant and also a higher sensitivity to SDS. Its nodulation phenotype on <italic>Lotus</italic> showed the induction of small and colorless 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 <italic>mll2707</italic> and <italic>mlr0970</italic> genes, were predicted as potential Tat substrates in <italic>M. loti</italic> MAFF303099. The transcriptional expression of <italic>mll2707</italic> and <italic>mlr0970</italic> genes was analyzed under different oxygen growth conditions. The <italic>mll2707</italic> gene was expressed constitutively, while the expression of the <italic>mlr0970</italic> gene was only detected under anaerobic and microaerophilic <italic>in vitro</italic> conditions. Both genes were down-regulated in the <italic>tatC</italic> mutant strain. <italic>mll2707</italic> and <italic>mlr0970</italic> mRNAs from the wild-type strain were detected in nodules. Using a translational reporter peptide fusion, we found that the Mll2707 protein was only detectable in the wild-type strain. On the other hand, although Mlr0970 protein was detected in wild-type and <italic>tatC</italic> mutant strains, its association with the membrane was favored in the wild-type strain. The <italic>tatC</italic> and the <italic>mll2707</italic> mutant strains were affected in the cytochrome c oxidase activity. These results confirm that Mll2707 is required for cytochrome c-dependent respiration and that Tat functionality is required for the correct activity of Mll2707. The <italic>mll2707</italic> mutant strain showed a nodulation phenotype similar to the <italic>tatC</italic> mutant strain, although it presented only a slight difference in comparison with wild-type strain in terms of nodule occupation. No defective phenotype was observed in the nodulation with the <italic>mlr0970</italic> mutant strain. These results indicate that, of the two Rieske Fe/S proteins coded by <italic>M. loti</italic> MAFF303099, only Mll2707 expression is required for the induction of effective nodules, and that the functionality of the Tat system is necessary not only for the correct function of this protein, but also for some other protein required in an earlier stage of the nodulation process.</p>