Tunable and Orthogonal ECF and Anti-σ Threshold Gates for Temporal Control of Heterologous Gene Expression

Precise timing of gene expression is a common feature in natural regulatory networks but is rarely implemented in synthetic pathways, where static control often limits performance. Here, we expand the use of bacterial extracytoplasmic function (ECF) σ factors by integrating their cognate anti-σ factors to create tunable threshold-gated circuits. Anti-σ overexpression has previously limited the utility of such systems due to growth inhibition. We address this by combining targeted truncations of membrane domains with chromosomal integration, yielding a library of ECF/anti-σ pairs that maintain function while minimizing toxicity. In Escherichia coli, these circuits can enable sharper OFF/ON switching, greater dynamic range, and tunable temporal delays compared with ECF-only cascades. In the best case, single-step threshold gates achieve up to 2100-fold induction with delays spanning minutes to hours. Extending the design to two steps enables programmable cascading of gene expression, with delays ranging from 30 to 409 min, although the dynamic range is generally reduced relative to single-step circuits. In the best-performing designs, matching input-output characteristics of promoters preserves dynamic range across cascade levels. Mathematical modeling supports these findings and highlights σ/anti-σ binding affinity as a key parameter for achieving high performance in longer cascades. Together, these results provide design principles for orthogonal, lower-burden timing circuits that enable controlled, sequential gene expression with minimal intervention in synthetic pathways and highlight continuing limitations of these systems.