Feedback Control of Two-Component Regulatory Systems

Abstract

Two-component systems are a dominant form of bacterial signal transduction. The prototypical two-component system consists of a sensor that responds to a specific input(s) by modifying the output of a cognate regulator. Because the output of a two-component system is the amount of phosphorylated regulator, feedback mechanisms may alter the amount of regulator, and/or modify the ability of a sensor or other proteins to alter the phosphorylation state of the regulator. Two-component systems may display intrinsic feedback whereby the amount of phosphorylated regulator changes under constant inducing conditions and without the participation of additional proteins. Feedback control allows a two-component system to achieve particular steady-state levels, to reach a given steady state with distinct dynamics, to express coregulated genes in a given order, and to activate a regulator to different extents, depending on the signal acting on the sensor.

Keywords: expression dynamics; phosphorylation; signal access; transcription surge.

Figure 1

Architectural classes of two-component signal transduction systems, and of signaling pathways using two-component system proteins. (a) In classical systems, the sensor harboring an input domain and a histidine kinase (HK) domain responds to an input by autophosphorylating at a conserved histidine residue. The phosphorylated sensor serves as a phosphoryl donor to the regulator harboring a response regulator (RR) domain and an output domain. The phosphorylated regulator generates an output. (b) A hybrid system harbors all the domains described for the classical system in panel a, in a single polypeptide. (c) In a phosphorelay, there are four consecutive phosphotransfer events, starting when a sensor responds to an input by autophosphorylating from ATP at a conserved histidine residue. The phosphoryl group is then transferred to a conserved aspartate in the RR domain and from there to a histidine residue in the histidine-containing phosphotransferase domain, and ultimately to an RR domain, which upon phosphorylation modifies the output domain to generate an output. The sequential phosphotransfer may occur between different domains of a single polypeptide, as depicted here, or between separate proteins harboring these domains. (d) In a convergent signaling pathway, distinct signals activate different sensors that modify the phosphorylation state of a single regulator to generate an output. The phosphorylation state of a regulator may also be modified by metabolites such as acetyl phosphate. (e) In a divergent signaling pathway, a sensor responds to a signal by modifying the phosphorylation state of two regulators, thereby generating two different outputs.

Figure 2

Feedback control of two-component systems. (a) Extrinsic control of two-component systems by a regulator governing transcription of the genes specifying the sensor and regulator, and by an Effector encoded by a gene under transcriptional control of the regulator. The Effector may affect input access to the sensor (42, 68, 80), bind to the sensor modifying its enzymatic activities (36, 40, 52, 82, 86), or modify the phosphorylation state of the regulator in a direct or indirect fashion (104). Effectors may act at other levels and are not necessarily proteins. (b) Intrinsic feedback by a sensor (from left to right) that responds to an input by binding to ATP, autophosphorylating at a histidine residue, and leaving ADP in the nucleotide-binding pocket. The regulator (RR) phosphorylates from the phosphorylated sensor. The ADP-bound sensor is in the phosphatase mode that promotes dephosphorylation of the phosphorylated regulator. The inset graph depicts the changes in the levels of phosphorylated regulator when an organism is switched from noninducing to inducing conditions for the sensor. Abbreviation: HK, histidine kinase.

Figure 3

Feedback mechanisms altering sensor access of an inducing signal, a repressing signal, and a protein with regulatory properties. (a) Under noninducing conditions for the sensor PmrB, the regulator PmrA is not active and the outermost part of the outer membrane is negatively charged owing to the phosphate residues in the lipopolysaccharide. The positively charged Fe³⁺ binds to the negatively charged outer membrane, which increases Fe³⁺ levels in the periplasm, where it serves as an inducing ligand for PmrB and results in a superactive PmrA. The covalent modification of the negative charges in the lipopolysaccharide by PmrA-activated gene products reduces the negative charge of the outer membrane, which decreases Fe³⁺ binding, further diminishing access to PmrB and activation of PmrA. (b) The sensor PhoQ is activated when periplasmic Mg²⁺ levels are low, resulting in phosphorylation of the regulator PhoP and transcription of PhoP-activated genes. The PhoP-activated mgtA gene specifies a Mg²⁺ transporter that imports Mg²⁺ from the periplasm into the cytoplasm, thereby removing an inhibitory signal for PhoQ. Thus, MgtA increases the levels of phosphorylated PhoP. (c) Misfolded proteins in the periplasm activate the sensor CpxA, favoring phosphorylation of the regulator CpxR and transcription of the CpxR-activated genes, including cpxP. The CpxP protein binds to misfolded proteins in the periplasm; as the levels of these proteins decrease, CpxP binds to the CpxA, decreasing its activation.