Recombinase

Recombinase is an enzyme that recognizes specific DNA sequences (recombination sites) and catalyzes strand exchange between them, producing defined genetic rearrangements without requiring DNA synthesis or high-energy cofactors ¹.

How It Works

Site-specific recombinases fall into two families: tyrosine recombinases (e.g., Cre, FLP) and serine recombinases (e.g., Bxb1, phiC31 integrases). Tyrosine recombinases catalyze reversible reactions between identical recognition sites (e.g., Cre recombines loxP sites). Depending on the relative orientation of two sites, recombination produces excision (direct repeats), inversion (inverted repeats), or intermolecular integration.

Serine integrases are particularly valuable for synthetic biology because they catalyze unidirectional recombination between non-identical attachment sites (attB and attP), producing hybrid sites (attL and attR) that are no longer substrates for the integrase. This irreversibility creates a permanent genetic switch — once flipped, the state is maintained through cell division without continued enzyme expression.

Recombinases enable genetic circuits with memory, sequential logic, and event counting. By nesting multiple orthogonal recombinase sites, designers can build state machines that record the order and identity of transient input signals as permanent DNA rearrangements, readable by sequencing or reporter gene expression.

Computational Considerations

Design automation tools enumerate all possible DNA rearrangement outcomes for a given arrangement of recombinase sites, verifying that the intended logic function is correctly implemented ². Graph-based models represent accessible states and transitions, enabling systematic exploration of circuit architectures that implement desired multi-input Boolean functions or sequential programs.

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