While there is only one architectural style for chemosensory systems, there are many different building materials. Each chemoreceptor senses one, or perhaps two related, types of chemicals. This means that if your cell wants to sense multiple chemicals in its environment, it needs a matching assortment of chemoreceptors. This can quickly get complicated. Some signals need to go to the flagella, some to the pili, and others to the transcriptional machinery to turn on or off genes. How can you keep the wires from getting crossed? The simplest approach is just to separate them, and that seems to be exactly what cells do. Chemoreceptors that signal to different systems have different lengths, which sorts them into separate arrays in the cell, as you can see in this Vibrio cholerae. The shorter arrays signal to the flagellar motor, and the longer array signals to a different target we have not yet identified. Even when multiple chemosensory systems signal to the same target, arrays that respond to different environmental cues are kept separate by length differences in the receptors (⇩).
