Prokaryotes are unicellular organisms that require sensory networks for their survival in rapidly changing habitats. In the course of evolution, transmembrane signaling systems have evolved to transmit signals from the extracellular environment across the cytoplasmic membrane into the cell. One-component signaling systems represent the oldest and simplest solution for such signal transmission, whereas two-component systems are evolutionarily younger (Ulrich et al., 2005). Although one-component systems are widely distributed among bacteria, only 3% are directly integrated into cytoplasmic membranes (Ulrich et al., 2005). A literature search revealed a non-exhaustive list of signaling molecules that includes ToxRS,