Characterization of the Sensory Rhodopsin II / Transducer II complex from Halobacterium salinarum

Motile bacteria respond to a variety of external stimuli by modulating the rotational direction of their flagella. Flagella driven motility is a necessity for coping with changing environments such as the decrease or increase of specific signaling molecules (chemotaxis), changes in the salinity (osmotaxis), temperature (thermotaxis), light intensity (phototaxis) and oxygen (aerotaxis) (1-3). Based on well-known chemotactic mechanisms in E. coli, signal transfer models of the events derived from chemoreceptor activation, achieve remarkable sensitivity, gain, dynamic range, and feedback control through the transmembrane signaling domain(4-10). Most of our knowledge on chemotaxis transducers is derived from the membrane-bound chemoreceptor proteins of bacteria (11,12). E. coli apparently evolved in an environment where sugars and amino acids served as energy sources. Therefore receptors to sense such substances are found in its inner membrane. Some ligands interact directly with the receptors in periplasmic space, such as serine with Tsr or aspartate with Tar, while ligand binding triggers a structural rearrangement of the binding protein, which allows the subsequent interaction with the receptor. Maltose, for example, is sensed by Tar through interaction with the maltose binding protein(13,14)