Gene Circuit Resource Competition — Ribosome Loading & Burden (2D)

Synthetic Circuit Load

Circuit A strength 40

Circuit B strength 0

Host Ribosome Pool

Total ribosomes R_tot 800

mRNA–ribosome affinity K 50

Live State

Free ribosomes

100%

Growth rate

100%

Circuit A output

0.0 /s

Circuit B output

0.0 /s

How it works

Every synthetic gene must be translated by the cell's own ribosomes — the same finite pool that translates the host's own housekeeping and growth genes. This is resource competition, the real mechanism behind the "burden" and "orthogonality" problems in synthetic biology: two circuits with no shared DNA or regulator can still couple to each other purely through the shared ribosome pool.

Each gene's mRNA competes for ribosomes as a saturable (Michaelis–Menten-like) binding process:

D  = (c_A + c_B + c_H) / K
R_free = R_tot / (1 + D)
flux_i = k · R_tot · c_i / (K + c_A + c_B + c_H)   for i in {A, B, host}
growth ∝ R_free / R_free(unloaded)

where c_A, c_B are the synthetic circuits' promoter/RBS strengths (your sliders), c_H is the host's fixed housekeeping demand, K is the mRNA–ribosome affinity (lower K = tighter binding = fiercer competition), and R_tot is the total ribosome count. The bound fraction for gene i simplifies algebraically to c_i / (K + c_A + c_B + c_H) — a standard multi-ligand competitive-binding result — which is what the flux line above and the particle-routing below both use.

Circuit A / B strength — raising one circuit's expression increases its mRNA's demand on the shared pool.
R_tot — a bigger ribosome pool (e.g. a faster-growing host strain) buffers against burden.
Affinity K — a stronger ribosome-binding site (lower K) wins more of the pool per unit of mRNA, starving everything else, including the cell's own growth machinery.

Watch what happens to Circuit B's output and the growth rate as you raise Circuit A alone — neither shares a promoter, operator, or transcription factor with A, yet both fall. That indirect coupling through a shared, finite resource is exactly what makes "orthogonal" synthetic circuits behave non-orthogonally inside a real cell. Drag inside the top view to pan; scroll to zoom.

Free ribosome Bound → Circuit A Bound → Circuit B Bound → Host growth genes

drag to pan · scroll to zoom