Domain architecture and gene context of several transducers from Natronomas and Halobacterium. (A) Two groups of transducers (upper boxes) with distinct domain architectures and their adjacent genes (arrows and lower boxes) from Natronomonas (blue dots) and Halobacterium (red dots) are schematically represented. Transducers with a long extracellular domain between their two transmembrane domains (box TM2ED) are frequently involved in chemotaxis, and are cotranscribed with the genes for periplasmic substrate-binding proteins (box SBP). Several of the transducers with a short loop between their two transmembrane domains (short-loop transducers, box TM2SL) are cotranscribed with retinal-containing photoreceptors (box RP) or distant homologs thereof. Experimental environmental response data are indicated by colored arrows (green: chemotaxis [Kokoeva et al. 2002]; blue: blue-light phototaxis [Seidel et al. 1995; Zhang et al. 1996]; orange: orange-light phototaxis [Yao and Spudich 1992]). The existence of orthologous gene pairs in Haloarcula is indicated by asterisks. (B) The interaction between a short-loop transducer (TM2SL) and a retinal protein (RP) occurs within the membrane. The interaction between a chemotactic extracellular-domain transducer (TM2ED) with a lipid-anchored (yellow box) periplasmic substrate-binding protein (SBP) may occur outside of the membrane.
