There are four conserved aspartate residues within the amino acid sequence of AroR (Fig. 2b), with aspartate 58 selleck compound residue predicted to be the most likely site of transphosphorylation. The receiver
domain is linked to an AAA+ATPase domain that precedes the DNA-binding domain at the C-terminus. The presence of an AAA+ATPase domain is indicative of a transcription factor activity associated with the activation of σ54 promoters. In analogous response regulators from the NtrC/DctD family, ATPase activity is coupled to a hexameric or a heptameric ring assembly that is required for the formation of an open RNA polymerase complex at the initiation of transcription (Gao & Stock, 2009). Furthermore, AroR sequence analysis shows the presence of a highly conserved ESELFGHEKGAFTGA
sequence motif that is essential for binding to the σ-factor of the σ54-RNAP (Yan & Kustu, 1999; Xu & Hoover, 2001; Bordes et al., 2003). We have previously detected a putative σ54-like promoter region upstream of aroB, and in a recent study of H. arsenicoxydans, it was shown, through transposon insertions, that alternative N sigma factor (σ54) of RNA polymerase is involved in the control of the arsenite oxidase gene expression (Koechler et al., 2010). aroR- and aroS-like genes appear to be conserved within gene clusters associated with arsenite oxidation (Fig. 1a). However, Akt inhibitor in members of the Alphaproteobacteria that include NT-26, the aroR and aroS genes are in the same Dichloromethane dehalogenase orientation as the arsenite oxidase genes,
whereas in members of the Betaproteobacteria, they are in the opposite orientation with a gene involved in oxyanion binding or phosphate/phosphonate transport in between them (Fig. 1a). Both AroS and AroR share high sequence similarity (∼80% identity) to analogous proteins from A. tumefaciens and O. tritici, with sequence similarities declining significantly to the next closest sequence homologues from Xanthobacter autotrophicus exhibiting sequence identities of 43% and 56% for AroS and AroR, respectively. In all the other identified organisms, which have homologous proteins, sequence identities range from approximately 38% to 23% for AroS-like proteins and 43% to 38% for AroR-like proteins, with significantly higher sequence conservation of AroR compared with that of AroS, possibly reflecting differences between various stimuli activating these sensors. The arsenite oxidase gene cluster consisting of aroB, aroA, cytC and moeA1 encodes two transcripts, one transcript that is constitutive and only contains cytC and moeA1 and another transcript that is inducible with arsenite and that contains all the genes and that is most likely regulated through an involvement of a putative σ54-like promoter upstream of aroB (Santini et al., 2007).