Deviations from the Rule: Orphan and Atypical Response Regulators
from: Two-Component Systems in Bacteria (Edited by: Roy Gross and Dagmar Beier). Caister Academic Press, U.K. (2012)
In a classical two-component system a response regulator is activated via phosphoryl group transfer from a cognate histidine kinase in order to elicit a cellular output response to a specific stimulus. However, it is increasingly recognized that response regulator proteins do not necessarily rely on this standard activation mode. The so-called orphan response regulators lack a cognate histidine kinase as phosphorylation partner. In many cases these proteins retain their function upon substitution of the phosphate-accepting aspartic acid residue in the receiver domain suggesting phosphorylation-independent mechanisms for output response control. The concept of unorthodox signaling to and activation of a response regulator is supported by the occurrence of atypical response regulators in several bacteria. Atypical response regulators comprise degenerate receiver domains with conserved residues required for the phosphotransfer reaction and signal propagation within the response regulator protein being absent, which certainly precludes canonical receiver phosphorylation. Nevertheless, atypical response regulators are involved in various cellular functions including vegetative cell growth, differentiation, secondary metabolism and virulence read more ...