Figure 1.

Modular scheme of the basic two component system (TCS) and the phosphorelay system (PS). The sensor domain (sensor), the histidine kinase phosphoacceptor domain (HisKA), the histidine-like ATPase domain (HATPase), the signal receiver domain (REC), the histidine-containing phosphotransfer domain (HPt), and the effector domain (effector) are shown in different colors. The phosphoryl group transfer is indicated by arrows. The PS includes additional regulatory steps, the phosphoryl group is transferred from the HisKA to a REC domain of the hybrid histidine kinase and subsequently transferred via a phosphotransfer protein to the response regulator (modified from Catlett et al. 2003).