Now that your cell can move, it needs to figure out which way to go. Perhaps it should take a sniff. Chemosensory systems are ancient (they were already present in the common ancestor of bacteria and archaea) and widespread, reflecting their great utility. Chemosensory systems are two-component signaling systems. The first component is a receptor that binds a specific chemical, such as a sugar or amino acid. The binding results in a conformational change that propagates down the long receptor (⇩), turning off a kinase bound at the other end that controls the state of the second component: a response regulator. These response regulator proteins then carry the signal elsewhere in the cell by diffusion.
For chemotaxis (“taxis,” or ordered movement, in response to chemicals) the signal is carried to the cell’s motility machinery, the flagellar motor or type IV pili. Phosphorylated response regulators bind the flagellar motor, switching the direction of rotation. This produces different results depending on the pattern of flagella on the cell. If there is a single flagellum, as on this Shewanella oneidensis cell, it switches the flagellum between pushing and pulling the cell body, reorienting the cell in the process. In peritrichously-flagellated cells like Escherichia coli, it brings the flagella into and out of a bundle. Remember that bundled flagella drive the cell forward in straight “runs” and dissociated flagella “tumble” the cell to try a new direction. (You can watch a video of this on Howard Berg’s website.) Constant feedback from the chemosensory system switches the balance of phosphorylated/unphosphorylated response regulators and therefore keeps the cell heading in the general direction of an attractant chemical cue, or away from a repellant.
Chemosensory systems form arrays containing many copies of the proteins. As in this S. oneidensis cell, arrays are usually located near the flagella they control, with the tips of the chemoreceptors sticking through the membrane into the periplasm or extracellular space where they can detect signals from the environment. At the other end of the array, the associated kinases interact with the response regulators. These kinases, along with an additional structural protein that helps organize the array, form a layer that we see as a dense line in the cytoplasm.