codon-constrain

codon-constrain is a dependency-free-runtime synonymous codon optimizer. It maximizes the sum of log relative codon adaptiveness for bundled E. coli or human profiles while preserving translation and jointly enforcing whole-construct GC, forbidden motifs on either strand, homopolymer length, and fixed DNA flanks.

Install

python -m pip install codon-constrain

For development:

python -m pip install -e '.[dev]'
pytest --cov=codon_constrain --cov-branch --cov-fail-under=100
ruff check .

Quickstart

Given enzyme.faa:

>enzyme
KK

run:

codon-constrain enzyme.faa \
  --output optimized.fasta \
  --report report.json \
  --host ecoli \
  --input-type protein \
  --gc-min 0.30 --gc-max 0.80 \
  --forbid GAATTC \
  --homopolymer-max 3

The FASTA contains the complete flanked construct. The JSON records translation preservation, the CAI-like geometric mean of relative adaptiveness, GC fraction, constraint checks, whether the solution is exact, and the unconstrained greedy baseline.

Python API:

from codon_constrain import Constraints, optimize

result = optimize(
    "KK",
    host="ecoli",
    input_type="protein",
    constraints=Constraints(homopolymer_max=3),
)
assert result.coding_sequence == "AAGAAA"
assert result.report["translation_preserved"]

Coding-DNA FASTA is also accepted with --input-type dna (or auto when unambiguous). Stop codons and unsupported residues are rejected with position-aware messages.

Algorithm and exactness

flowchart LR; A[protein or coding DNA] --> S[DP states: GC count × suffix × homopolymer run]; S --> T[extend by synonymous codons]; T --> C{constraints: GC, motifs, flanks}; C -->|feasible| B[max log adaptiveness]; B --> R[traceback: optimal construct + report]

At each amino-acid position, dynamic programming extends every reachable finite state by one synonymous codon. A state consists of:

• total GC count;
• the suffix needed to recognize a forbidden motif at the next base;
• the final base and current homopolymer run length.

Motifs are expanded to include their reverse complements. Fixed 5' and 3' flanks pass through the same state machine, so violations spanning a flank/coding boundary are detected. For identical states, only the highest-scoring prefix is retained because all future feasibility and score contributions depend solely on that state. Therefore the default unpruned mode proves the maximum score over these represented finite-state constraints; this is dynamic programming, not full-sequence enumeration. --beam-width N retains only the best N states per residue to cap memory, and the report honestly marks that result non-exact.

The objective is

sum(log(relative_adaptiveness(codon)))

and its reported CAI-like value is the geometric mean, exp(log_score / codon_count). Bundled weights are normalized within each synonymous family; they are useful deterministic host profiles rather than an expression guarantee.

Reproducible capability evidence

The test suite contains an exhaustive tiny oracle for protein KTL: it enumerates only that test's 36 synonymous sequences and verifies that exact DP returns the same feasible optimum under GC, motif, and homopolymer constraints. This independently checks the optimality recurrence.

A deterministic material improvement over greedy selection is exercised end to end:

protein:             KK
host:                E. coli
greedy best codons:  AAAAAA   (violates homopolymer_max=3)
exact DP result:     AAGAAA   (translation KK; feasible)

Run the evidence:

pytest tests/test_optimizer.py::test_exact_matches_tiny_brute_force_oracle \
       tests/test_optimizer.py::test_greedy_failure_dp_success_is_deterministic_capability_example \
       tests/test_cli.py::test_installed_module_cli_end_to_end_from_clean_directory

Mutation testing

From the repository root, reproduce the mutation run with:

source .venv/bin/activate
mutmut run
mutmut results

The run generated 807 mutants and killed 768 (95.17%). The remaining 39 were individually reviewed as behavior-equivalent under the supported contract, not missed mutants. There were zero suspicious results and zero timeouts. Observable CLI help changes were killed rather than classified as equivalent.

Limitations

• Optimization models synonymous codon choice only; it does not predict expression, mRNA folding, ribosome traffic, splicing, synthesis success, or biological safety.
• Inputs must use the 20 canonical amino acids or stop-free ACGT coding DNA. Ambiguous bases, stop codons, and selenocysteine are unsupported.
• GC is enforced globally across the complete flanked construct, not in sliding windows.
• Long motifs and broad GC ranges can produce many states. Beam search bounds memory but removes the optimality proof.
• Host tables are built-in relative profiles and are not tissue-, strain-, condition-, or gene-specific. Validate designs experimentally and against current domain-specific requirements.