1, YP_001941631 1, YP_001941634 1, YP_001585641 1),

R li

1, YP_001941631.1, YP_001941634.1, YP_001585641.1),

R. litoralis (ZP_02142508.1), Pseudomonas sp.TS44 (ACB05952.1), C. phaeobacteroides (YP_001960746.1) and C. aggregans (YP_002461760.1). Remarkably, a multiple alignment of amino acid sequences revealed that AoxR shares significant homology with a number of σ54 RNA polymerase transcriptional activators, i.e. 35.96% identity with ZraR and 35.26% identity with AtoC from E. coli K12. AoxR contains three conserved domains shared by most Enhancer Binding Proteins (EBP), namely a N-terminal response regulator receiver domain (amino acids 18-130), a central σ54 interaction domain (amino acids 147-368) common to all σ54 dependent EBPs (Pfam E-value 10-116; http://​www.​sanger.​ac.​uk/​cgi-bin/​Pfam/​getacc?​PF00158) and a C-terminal DNA binding helix-turn-helix SAHA HDAC ic50 BTK inhibitor (HTF) domain (amino acids 421-463) enable to bind to specific upstream activation sequences [20]. AoxR shares similarities with several EBPs of σ54 essential for the formation of an open complex formation during σ54-dependent transcriptional initiation, in particular the σ54 activator sequence GAFTGA loop 1 which directly binds to σ54 conserved region III (Figure 6) [21]. Taken together, these observations strongly suggest that AoxR interacts directly with RpoN to initiate the transcription of aoxAB operon in H. arsenicoxydans. Figure 6 Amino acids conservation between σ 54 Enhancer Binding Proteins (EBP) and AoxR. Sequence

alignment was performed with ClustalW. The conserved amino acids are presented with a blue background Branched chain aminotransferase and the blue intensity reflects sequence similarities.

Only the central binding domain is indicated. Region CI has remarkable similarity to the consensus glycine-rich flexible loop motif (Walker A – consensus motif GxxGxGK), and also contains hydrophobic residues. Region CII is hydrophobic. The region CIII is predicted to fold into two alpha helices separated by a turn. This region is involved in a specific interaction between the EBP and the Eσ54 required for open promoter complex formation via the GAFTGA motif. Region CIV is rich in glycine, negatively charged and contains a consensus sequence of 4 aliphatic residues followed by 2 negatively charged residues (Walker B – consensus motif TVFLDE); in contrast, CVI is positively charged and is rich in aromatic residues and proline. Region CV is found about 80 amino acids away from region CI, and has a consensus sequence QakLLRVLqe. Finally, region CVII has a core of eight highly conserved amino acids. Sequence informations of other genes were obtained from Colibri database (Institut Pasteur, Paris). Discussion Despite many works devoted to arsenic metabolism in microorganisms, little is known about the regulation of arsenite oxidase activity. In the present study, the combination of transcriptomic, genetic and molecular data provided a comprehensive view of the role of various proteins in the control of arsenite oxidation in H. arsenicoxydans (Figure 7).

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