dR, Delta-rhodopsin, is a light-driven proton transporter found in the archaebacterium Haloterrigena turkmenica. It functions to generate the 'proton motive force' (PMF), which is an intracellular energy supply for various bio-reactions in all living organisms. For example, nerve actions are carried out by the PMF, various cellular substrates are transported by the PMF, bacterial flagella are driven by the PMF, and ATP is synthesized by the PMF across the cytoplasmic membranes of prokaryotic cells and the inner mitochondrial membranes of eukaryotic cells. The PMF is also generated from light energy by the thylakoid membranes of chloroplasts and the cytoplasmic membranes of hyperhalophilic bacteria. Heterologous expression of dR can produce a light-driven PMF photogenerating ability, like that in chloroplasts. For example, light-driven activation of dR targeted to the mitochondrial membrane results in proton transport from the mitochondrial matrix to the inner mitochondrial membrane space, thereby increasing the PMF across the mitochondrial membrane. (PMID:23567447.)