Signal transduction response regulator, propionate catabolism, transcriptional regulator PrpR <p>Two-component signal transduction systems enable bacteria to sense, respond, and adapt to a wide range of environments, stressors, and growth conditions [<cite idref="PUB00042804"/>]. Some bacteria can contain up to as many as 200 two-component systems that need tight regulation to prevent unwanted cross-talk [<cite idref="PUB00042805"/>]. These pathways have been adapted to response to a wide variety of stimuli, including nutrients, cellular redox state, changes in osmolarity, quorum signals, antibiotics, and more [<cite idref="PUB00010651"/>]. Two-component systems are comprised of a sensor histidine kinase (HK) and its cognate response regulator (RR) [<cite idref="PUB00011096"/>]. The HK catalyses its own auto-phosphorylation followed by the transfer of the phosphoryl group to the receiver domain on RR; phosphorylation of the RR usually activates an attached output domain, which can then effect changes in cellular physiology, often by regulating gene expression. Some HK are bifunctional, catalysing both the phosphorylation and dephosphorylation of their cognate RR. The input stimuli can regulate either the kinase or phosphatase activity of the bifunctional HK.</p><p>A variant of the two-component system is the phospho-relay system. Here a hybrid HK auto-phosphorylates and then transfers the phosphoryl group to an internal receiver domain, rather than to a separate RR protein. The phosphoryl group is then shuttled to histidine phosphotransferase (HPT) and subsequently to a terminal RR, which can evoke the desired response [<cite idref="PUB00042806"/>, <cite idref="PUB00042807"/>].</p><p>This entry represents the signal transduction response regulator PrpR. At least five distinct pathways exist for the catabolism of propionate by way of propionyl-CoA. Members of this family represent the transcriptional regulatory protein PrpR which is, in most cases, divergently transcribed from the operon that encodes the genes involved in the methylcitric acid cycle of propionate catabolism. This protein is required for the expression of the proteins involved in this pathway [<cite idref="PUB00020194"/>]. 2-methylcitric acid, an intermediate in this pathway, has been proposed to be a co-activator of PrpR [<cite idref="PUB00033359"/>].</p>