In an ever-changing environment, it is essential that organisms are able to sense these changes and to respond appropriately. Possible responses include alterations in gene expression and/or active movement towards or away from an environment. Most sensory pathways in eukaryotic organisms rely on serine, threonine or tyrosine protein kinases, whereas the most common sensory pathways in prokaryotes use a HAP (Histidine-Aspartate Phosphorelay) system. HAP systems have at least two components-a dimeric HPK (Histidine Protein Kinase) and a RR (Response Regulator). HAP systems are also found in many lower eukaryotes. Bacteria can sense a vast range of environmental signals, from the concentrations of nutrients and toxins to oxygen levels, pH, osmolarity and the intensity and wavelength of light. Bacteria are able [...]