Bacterial transmembrane signalling systems and their engineering for biosensing

Abstract

Every living cell possesses numerous transmembrane signalling systems that receive chemical and physical stimuli from the environment and transduce this information into an intracellular signal that triggers some form of cellular response. As unicellular organisms, bacteria require these systems for survival in rapidly changing environments. The receptors themselves act as 'sensory organs', while subsequent signalling circuits can be regarded as forming a 'neural network' that is involved in decision making, adaptation and ultimately in ensuring survival. Bacteria serve as useful biosensors in industry and clinical diagnostics, in addition to producing drugs for therapeutic purposes. Therefore, there is a great demand for engineered bacterial strains that contain transmembrane signalling systems with high molecular specificity, sensitivity and dose dependency. In this review, we address the complexity of transmembrane signalling systems and discuss principles to rewire receptors and their signalling outputs.

Keywords: CadC; KdpD; ToxR; YehU; signal transduction; two-component system.

Figure 1.

Schematic presentation of the major types of transmembrane signalling systems in bacteria. One-component signalling systems, consisting of sensor and DNA-binding domain (yellow), two-component systems with a membrane-integrated histidine kinase (HK) and a response regulator (RR) (green), and extracytoplasmic function (ECF) sigma factors (σ) that will be released from the anti-sigma factor (anti-σ) upon stimulus perception (blue).

Figure 2.

The complex regulation of CadC, a one-component system representative. CadC is the regulator of the cadBA operon encoding the lysine decarboxylase CadA and the lysine/cadaverine antiporter CadB. Under non-inducing conditions, the lysine-specific transporter LysP inhibits CadC. When cells are exposed to low pH in the presence of lysine, the interaction between LysP and CadC is weakened, rendering CadC susceptible for protonation and transcriptional activation. The end-product of decarboxylation, cadaverine, binds to CadC and thereby inactivates this receptor.

Figure 3.

Schematic of the Kdp regulation system. The bifunctional receptor histidine kinase KdpD acts as both an autokinase (including phosphotransferase) and phosphatase for the response regulator KdpE. Phosphorylated KdpE activates expression of the genes encoding the high-affinity K⁺ transporter KdpFABC. KdpD autokinase activity depends on the external K⁺ concentration, and the phosphatase activity is influenced by the internal K⁺ concentration.

Figure 4.

Schematic of the pyruvate-sensing BtsS/BtsR/YpdA/YpdB network. The scheme summarizes the regulatory network associated with signal transduction by the BtsS/BtsR system, the influence of the YpdA/YpdB system and the global regulators cAMP-CRP, LeuO and CsrA.

Figure 5.

Principles to rewire transmembrane signalling systems. Membrane-integrated methyl-accepting chemotaxis proteins (MCPs, left part) are generally composed of two modules: an input domain in the periplasm (PP) and in the cytoplasmic membrane (CM), which is responsible for ligand binding and signal transduction, and an output domain in the cytoplasm (CP), which induces a cellular response. Both domains are connected by a linker domain. Whereas input domains are highly diverse, variation in the output domains is rather limited. There are three common schemes for an output: nucleotide cyclase activity (NTP = nucleotide triphosphate → cNMP = cyclic nucleotide monophosphate) (outermost left); alterations of the direction of the flagellar motor (CCW = counter clockwise, CW = clockwise rotation) including the formation of a ternary complex between MCPs/CheW(W)/CheA(A) and (de-)phosphorylation of CheY(Y) (innermost left), and transcriptional regulation (right). Sensor kinases perceive a stimulus and transduce the signal via phosphorylation to a response regulator that acts as transcription factor of natural or reporter genes. The modular design of transmembrane signalling systems allows the generation of chimeric receptors in which the input, the linker or the output domain is replaced (domain colour switch). Sensor kinases can be rewired by amino acid replacement (blue/green stripes) to allow activation of a non-cognate response regulator.