Biochemical mechanisms of a simple behavioral reaction

In active search for conditions for optimal phototrophic growth, halobacter ia respond to light and to a variety Of Other environmental stimuli by taxi s. Light reception is mediated by sensory rhodopsins, seven helix transmemb rane retinal proteins that are stably complexed to specific MCP-related tra nsducer proteins. Photoisomerization of the retinal chromophore generates a steric signal which activates the sensory rhodopsin-transducer-complex. Co mplex activation is relayed to a two component signaling system and transie ntly changes the cytoplasmic concentration of fumarate which, together with the small cytoplasmic protein CheY controls flagellar motor switching. Dua l signaling by CheY and fumarate was also found in Escherichia coli where i t mediates metabolic signaling and taxis in cells with disabled two-compone nt system. Sensory integration of different environmental stimuli occurs at the level of the signaling domains of several stimulus-specific transducer s constructed through combination of appropriate molecular modules. Irrespe ctive of the complexity of the signaling network, the response to light is quantitatively predictable and stochastic. Its kinetic analysis suggests th at time-dependent changes in the activity of signaling components can be re solved even in the context of a complex molecular network.