Sensing Subsystem

The sensing subsystem of a synthetic cell is responsible for detecting signals in the cell's environment and converting them into intracellular biochemical responses. This page describes the molecular mechanisms used for sensing, with emphasis on implementations that have been demonstrated in cell-free or synthetic cell contexts.

Sensing Mechanisms

A variety of signals can be detected within a cellular environment using different biomolecular mechanisms. The common principle underlying most sensors is a conformational change that occurs when a molecule binds to a protein, converting ligand occupancy into a change in protein activity.

Inducible Transcription Factors

One large class of sensors are inducible transcription factors. These proteins change their regulatory activity upon binding a specific small molecule, which modulates their ability to interact with DNA. In some cases, the inducer is required for the transcription factor to bind DNA and exert either repression or activation (positive inducers). In other cases, binding of the inducer inhibits DNA binding or regulatory activity, for example by altering protein conformation or occluding DNA-binding domains (negative inducers, whose presence relieves repression or activation). Canonical examples include the LacI–IPTG and TetR–aTc systems, both of which operate through negative induction, producing gene expression when the inducer is present.

Inducible transcription factors are widely used in synthetic cell systems because they are well-characterized, modular, and operate entirely within the cell-free transcription–translation machinery without requiring membrane integration. Their primary limitation is that the inducer must be able to cross the synthetic cell membrane, either by passive diffusion or via a transport mechanism.

Two-Component Signaling Systems

A more sophisticated sensing architecture is the two-component signaling system, which allows detection of extracellular signals without requiring the signal molecule to enter the cell. A typical two-component system consists of a transmembrane sensor kinase and a cytoplasmic response regulator. The extracellular domain of the sensor kinase binds a signaling molecule, triggering a conformational change that leads to autophosphorylation of the kinase. The activated kinase then transfers the phosphate group to the response regulator, which in turn binds DNA or carries out another downstream function.

From a systems perspective, this architecture implements a modular sensor–transducer block that maps an extracellular input (ligand concentration) to an intracellular output (phosphorylated response regulator concentration). The separation between sensing, membrane transduction, and downstream response mirrors the structure of engineered feedback systems and enables two-component networks to serve as standardized interfaces between the environment and synthetic cell internal logic.

Demonstration: Transmembrane Signal Transduction in Synthetic Membranes (Kamat, 2023)

Reconstitution of two-component signaling across a synthetic membrane. (a) The NarX/NarL system couples nitrate sensing to reporter expression in the presence of a membrane mimetic. (b) Systematic omission experiments confirm that all components of the sensor are required for reporter expression. (c) Inclusion of synthetic lipid membranes (DMPC liposomes) enhances nitrate-dependent reporter expression. (d,e) Sensor output and fold change can be tuned by adjusting the NarX:NarL DNA ratio. Peruzzi et al., 2023, Figure 1.

Kamat's group at Northwestern University demonstrated the reconstitution of a bacterial two-component signaling system within synthetic lipid membranes, providing a bottom-up implementation of transmembrane signal transduction in a synthetic cell context. The authors reconstituted the NarX/NarL system, consisting of a transmembrane sensor kinase (NarX) embedded in a synthetic lipid bilayer and its cognate response regulator (NarL) encapsulated on the interior side of the membrane. Binding of nitrate to the extracellular domain of NarX triggered autophosphorylation of NarL, which in turn drove expression of a nanoluciferase reporter.

The work demonstrated that signal gain and dynamic range could be tuned by adjusting the NarX:NarL DNA ratio, trading off absolute signal level against sensitivity to nitrate. Selective insulation of signaling pathways was also shown by choosing orthogonal kinase–regulator pairs, pointing toward the possibility of multiplexed sensing with minimal crosstalk.

Sensing as Part of the Control Architecture

In the context of synthetic cell design, the sensing subsystem provides the input layer of a feedback control loop. Sensed signals are passed to computational elements (the Logic Subsystem or Regulation Subsystem) that compare them to reference states and generate commands for the Mechanical Actuation Subsystem or other effectors. Realizing this full loop within a synthetic cell requires that sensing, computation, and actuation be designed with compatible interfaces — in particular, that the output of a sensor (typically a change in transcription factor or response regulator activity) be in a form that can drive downstream genetic circuits.

Key open challenges for sensing subsystem design include:

• Membrane integration: embedding transmembrane sensor proteins into synthetic cell membranes with correct topology and sufficient copy number for reliable detection.

• Signal range and sensitivity: biological sensors are typically optimized for the concentration ranges found in living cells, which may not match the ranges relevant to synthetic cell applications.

• Orthogonality: operating multiple sensors simultaneously requires that they do not crosstalk — either through shared regulatory proteins or through metabolic load effects on the shared transcription–translation machinery.

• Dynamic range and adaptation: unlike electronic sensors, biomolecular sensors are subject to saturation, cooperativity, and adaptation effects that must be accounted for in circuit design.