Two-component regulatory systems may also be used to develop tightly regulated gene expression systems. Tightly regulated gene expression is important in engineering metabolic pathways to avoid leaky expression that may cause a metabolic burden to the microbial cell. Typical induction strategies include the use of isopropyl-β-d-thiogalactopyranoside (IPTG). However, IPTG is expensive and can be toxic to cells at high concentrations. An example of tightly regulated gene expression induced by an inexpensive substrate is the invertible promoter system. In this system, the promoter is active or ‘ON’ when the target substrate that serves as an inducer is present and ‘OFF’ (inverted orientation) when absent . Based on this invertible promoter system’s mechanism, the coupled sensing and regulating activities of TCRSs can be modified to achieve tightly regulated gene expression.