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The logic subsystem of a synthetic cell is responsible for processing sensed information and deciding on appropriate actions. This includes both instantaneous input–output computations (combinational logic) and time-dependent behaviors that depend on the history of inputs (memory and state). In synthetic cells, both functions are implemented using the same underlying molecular machinery — chemical reaction networks (CRNs), transcription factors, and DNA-modifying enzymes — rather than the digital circuits used in electronic systems.

== Computation ==

Most modern engineered control systems rely on digital computation, but biological control systems operate closer to analog computation. Chemical reaction networks provide a natural substrate for implementing dynamical behaviors: molecular concentrations play the role of state variables, reaction rates play the role of gains, and the network topology determines the input–output relationship of the circuit. A key advantage of the CRN formalism is that it connects directly to control theory, allowing standard feedback control objectives — reference tracking, disturbance rejection, robustness — to be mapped onto circuit designs and analyzed using established tools.

In addition to analog feedback computation, biological circuits can implement discrete event systems. Recombinase-based circuits are particularly well suited to this role: serine and tyrosine recombinases catalyze irreversible DNA inversions or excisions in response to specific inputs, producing permanent changes in gene expression state that are stable without continued energy input.

=== Recombinase-based state machines (Roquet et al., 2016) ===

Roquet and colleagues demonstrated synthetic recombinase-based state machines in living cells that record the order and combination of input signals as distinct DNA configurations, implementing a finite state machine with multiple stable states<ref name="Roquet2016">N. Roquet, A. P. Soleimany, A. C. Ferris, S. Aaronson, and T. K. Lu, [https://doi.org/10.1126/science.aad8559 Synthetic recombinase-based state machines in living cells]. ''Science'' 353(6297):aad8559, 2016.</ref>.

== Memory and state ==

Memory in a synthetic cell control system means the ability to store information about past events and use it to influence future behavior. This is essential for implementing hybrid and event-driven behaviors, where the appropriate response depends not just on the current input but on the history of inputs.

=== Rewritable digital memory (Bonnet et al., 2012) ===

Bonnet and colleagues showed that recombinases can be composed to create rewritable digital memory elements, enabling discrete, reversible transitions between well-defined genetic states in response to inputs<ref name="Bonnet2012">J. Bonnet, P. Subsoontorn, and D. Endy, [https://doi.org/10.1073/pnas.1202344109 Rewritable digital data storage in live cells via engineered control of recombination directionality]. ''Proceedings of the National Academy of Sciences'' 109(23):8884–8889, 2012.</ref>. This established the basic principle that DNA configuration can serve as a stable, readable memory medium in biological systems.

=== Temporal logic gate (Hsiao et al., 2016) ===

Hsiao and colleagues demonstrated a population-based temporal logic gate that uses recombinase-mediated DNA rearrangements to encode the order and timing of chemical events<ref name="Hsiao2016">V. Hsiao, Y. Hori, P. W. K. Rothemund, and R. M. Murray, [https://doi.org/10.15252/msb.20156663 A population-based temporal logic gate for timing and recording chemical events]. ''Molecular Systems Biology'' 12(5):869, 2016. DOI: 10.15252/msb.20156663</ref>. The system distinguishes between signals that arrive in different orders, producing different outputs depending on which input was seen first — a function not achievable with combinational logic alone.

=== Continuous event logging ===

More recent approaches extend DNA-based memory to continuous event logging. Shur and Murray introduced an architecture that records multiple chemical stimuli into a growing genomic array by combining serine integrases with CRISPR-dCas9-mediated site selection<ref name="Shur2021">A. S. Shur and R. M. Murray, [https://doi.org/10.1101/225151 Proof of concept continuous event logging in living cells]. bioRxiv, 2021. DOI: 10.1101/225151</ref>. The MEMOIR system uses CRISPR-mediated mutagenesis to stochastically encode cellular history into distributed genomic barcodes readable ''in situ''<ref name="Frieda2017">K. L. Frieda et al., [https://doi.org/10.1038/nature20777 Synthetic recording and in situ readout of lineage information in single cells]. ''Nature'' 541:107–111, 2017.</ref>.

== Logic in the control architecture ==

The logic subsystem occupies the central processing layer of the synthetic cell control architecture, sitting between the [[Sensing Subsystem]] (inputs) and the [[Mechanical Actuation Subsystem]] or gene expression outputs (actions). Key design considerations include:

* ''Analog vs. discrete'': CRN-based circuits implement graded, continuous responses well-suited to feedback regulation; recombinase-based circuits implement sharp, irreversible transitions well-suited to state machines and memory. Many applications will require both.

* ''Resource load'': logic circuits consume transcriptional and translational capacity from the shared [[Cytoplasm Subsystem]]. Complex circuits with many genes impose significant burden and must be designed with resource competition in mind.

* ''Composability'': circuits designed independently must be combined without unexpected interactions. Orthogonal transcription factors, insulated promoters, and contract-based design frameworks are tools for achieving this.

* ''Robustness'': biological logic circuits operate in a noisy, variable environment. Feedback is a primary tool for achieving robustness to molecular noise and load disturbances.

== References ==
<references />

[[Category:Subsystem]]

Logic Subsystem - Revision history