Crystal Structure and Regulation of the Citrus Pol III Repressor MAF1 by Auxin and Phosphorylation

MAF1 is the main RNA polymerase (Pol) III repressor that controls cell growth in eukaryotes. The Citrus ortholog, CsMAF1, was shown to restrict cell growth in citrus canker disease but its role in plant development and disease is still unclear. We solved the crystal structure of the globular core of CsMAF1, which reveals additional structural elements compared with the previously available structure of hMAF1, and explored the dynamics of its flexible regions not present in the structure. CsMAF1 accumulated in the nucleolus upon leaf excision, and this translocation was inhibited by auxin and by mutation of the PKA phosphorylation site, S45, to aspartate. Additionally, mTOR phosphorylated recombinant CsMAF1 and the mTOR inhibitor AZD8055 blocked canker formation in normal but not CsMAF1-silenced plants. These results indicate that the role of TOR on cell growth induced by Xanthomonas citri depends on CsMAF1 and that auxin controls CsMAF1 interaction with Pol III in citrus
This work was supported by Sa˜ o Paulo Research Foundation (FAPESP grant 2011/20468-1). C.E.B. and A.F.Z.N. received a fellowship from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).