The four rhodopsins of H. salinarum. The four rhodopsins of H. salinarum. The two ion pumps, BR and HR, convert light energy into a proton and a chloride gradient, respectively, which are utilized by the cell for its energy-requiring steps, e.g., ATP synthesis, metabolism, and/or ion transport. The two other retinylidene pigments, SRI and SRII, function as photoreceptors, which direct the bacteria to optimal light conditions. The external stimuli, light and chemoeffectors, activate their corresponding receptors, i.e., the SRs, SRI and SRII, or the chemoreceptors. The SRs form complexes with their homodimeric transducers, HtrI and HtrII. After the absorption of light, the signal is transferred to the cytoplasmic domain of the transducers, which consists of three parts. Directly following the two membrane-spanning helices are small domains that are specific for the Htrs. The methylation sites downstream are involved in processes of adaptation to constant stimuli. Finally, the signaling domain interacts with a His kinase (CheA), which then transfers this information via the phosphoregulator (CheY) to the flagellar motor switch. (The figure is reproduced from reference 526 with the kind permission of J. L. Spudich.)‏ Günter Schäfer et al. Microbiol. Mol. Biol. Rev. 1999; doi: