Bacterial signal transduction
Common to all bacterial signal transduction systems is the detection of a signal (input) and coupling this with an adaptive cellular response (output); however, bacteria employ diverse mechanisms for linking these events, ranging from single-domain transducers to several interacting proteins. The simplest and most abundant signal transduction systems are one-component systems that possess both sensory and regulatory properties on a single protein molecule (Ulrich et al., 2005). The most widely recognized signaling systems are two-component systems that utilize protein phosphorylation as a fundamental strategy for signaling (Hoch and Silhavy, 1995; Stock et al., 2000; Inouye and Dutta, 2003). The prototypical two-component systems consists of a membrane-bound, sensor histidine kinase and a cytoplasmic response regulator. The sensor kinase detects an environmental signal via its input domain(s). This triggers an ATP-dependent autophosphorylation of a conserved histidine residue within its carboxy-terminal transmitter domain. The response regulator then catalyzes the transfer of this phosphoryl group to a conserved aspartate residue within its receiver domain. Phosphorylation of the receiver domain activates the output domain(s), which effects a particular adaptive response, usually regulation of transcription (Parkinson and Kofoid, 1992). Phosphorelays extends the basic two-component them with the inclusion of additional histidine phosphotransfer (HPt) and aspartate containing receiver domains (Appleby et al., 1996, Hoch, 2000).