A Sigma Factor Toolbox Enables Orthogonal Gene Expression for Modular Control of Complex Pathways in Escherichia coli

The development of microorganisms as mini-factories for synthesis of various (bio)chemicals constitutes an interesting environmental friendly alternative to the more polluting petroleum-based chemical synthesis. Synthetic biology, that utilizes engineering principles to design novel biological systems may contribute significantly to the further development and success of this bio-based approach. This requires separate well-defined parts to combine and express complex genetic circuits in an independent and controllable manner. Here we contribute to the expansion of the synthetic biology toolbox for model organism Escherichia coli by creating orthogonal expression systems based on heterologous sigma factors. Furthermore, specific inhibition of sigma factor activity is obtained by introducing the corresponding anti-sigma factor. Finally, promoter libraries were generated for three sigma factors without loss of orthogonality, allowing a wide range of expression profiles and fine-tuning within a module and among modules.