Thus a slightly altered ligand accessibility may have evolved for the distinct clades (+)-JQ1 outlined here. The association of the Vch_cass2 gene with mobile DNA elements, also notably evident for its group of related homologs, emphasises the mechanism by which these binding modules can be laterally transferred between species. While the presence of a DNA-binding partner appears necessary for transcription regulation, we cannot rule out the possibility that the biological function of Cass2 itself may be to provide a self-contained low-level multidrug resistance system, capable of sequestering drugs and preventing them from reaching further intracellular targets. The role of cationic drugs in treatment of cholera and inhibition of cholera toxin-internalization has been previously reported. The depiction in this work of a novel effector domain capable of binding cationic compounds is therefore of immediate interest, given that these are encoded within the mobile integron gene cassette system. We have, however, noted surface features in the Cass2 structure consistent with a protein interaction site adjacent to the active-site cavity. We propose this to comprise a potential site for interaction of the effector-binding module with a specific DNAbinding domain, so as to mimic the organisation of the multidomain transcription regulators. This is congruent with the more general observation that two interacting prokaryotic proteins, not necessarily encoded by neighbouring genes, may be found fused as a single chain homolog in another organism. Such component proteins might be engaged in either direct physical interaction or an indirect functional association. Sequence searches were conducted to locate any likely companion module for Cass2 in V. cholerae; no sequence homolog of the singledomain protein MarA was found amongst gene cassettes from the same environmental isolate as Cass2. However, wider sequence searches across published Vibrio genomes do reveal the existence of BEZ235 single-domain homologs containing the helix-turn-helix motifs present in both MarA and Rob relatives. The overall structure of the Cass2 protein and its relationship to other members of the AraC/XylS and MerR family reinforces the notion that gene cassettes within integron arrays generally move and rearrange independently of one other. Given that many cassettes encode single small domain proteins, loss of intervening attC site sequences may lead to permanent fusion of gene cassettes so as to instead encode a multi-domain polypeptide that confers advantage. Our recovery of an independent single domain with effector-binding capacities is significant as a possible evolutionary precursor to the multi-domain transcription regulators, of which the AraC and MerR families are examples. Evidence for fusion events in the evolution of MerR regulators has previously been outlined. For example, the tipA gene of S. lividans encodes single and two domain gene products.