During signal transduction by two-component regulatory systems, sensor kinases detect and encode input information while response regulators (RRs) control output. Most receiver domains function as phosphorylation-mediated switches within RRs, but some transfer phosphoryl groups in multistep phosphorelays. Conserved features of receiver domain amino acid sequence correlate with structure and hence function. Receiver domains catalyze their own phosphorylation and dephosphorylation in reactions requiring a divalent cation. Molecular dynamics simulations are supplementing structural investigation of the conformational changes that underlie receiver domain switch function. As understanding of features shared by all receiver domains matures, factors conferring differences (e.g. in reaction rate or specificity) are receiving increased attention. Numerous examples of atypical receiver or pseudo-receiver domains that function without phosphorylation have recently been characterized.
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