Characterization of putative transcriptional regulator (PH0140) and its distal homologue.

In this study, a phylogenetic tree was constructed using 1854 sequences of various Lrp/AnsC (FFRPs) and ArsR proteins from pathogenic and non-pathogenic organisms. Despite having sequence similarities, FFRPs and ArsR proteins functioning differently as a transcriptional regulator and de-repressor in the presence of exogenous amino acids and metal ions, respectively. To understand these functional differences, the structures of various FFRPs and ArsR proteins (134 sequences) were modeled. Several ArsR proteins exhibited high similarity to the FFRPs while in few proteins, unusual structural folds were observed. However, the Helix-turn-Helix (HTH) domains are common among them and the ligand-binding domains are structurally dissimilar suggest the differences in their binding preferences. Despite low sequence conservation, most of these proteins revealed negatively charged surfaces in the active site pockets. Representative structures (PH0140 and TtArsR protein) from FFRPs and ArsR protein families were considered and evaluated for their functional differences using molecular modeling studies. Our earlier study has explained the binding preference of exogenous Tryptophan and the related transcriptional regulatory mechanism of PH0140 protein. In this study, a Cu2+ ion-induced de-repression mechanism of the TtArsR-DNA complex was characterized through docking and molecular dynamics. Further, the proteins were purified and their efficiency for sensing Tryptophan and Cu2+ ions were analyzed using cyclic voltammetry. Overall, the study explores the structural evolution and functional difference of FFRPs and ArsR proteins that present the possibilities of PH0140 and TtArsR as potential bio-sensory molecules.