One of the core assets of synthetic biology is the control over which protein is produced at which time and to what extent (also referred to as gene expression). By designing a specific promoter, which regulates the production of a protein from the regulated gene via mRNA, synthetic biology researchers can couple the activity of this gene to a specified signal. This promoter can for instance be responsive to cellular signaling activated in the presence of calcium ions. Then, cells perform a designated action through the inducible gene expression and its concomitant protein production, once calcium ions are introduced.
These inducible gene expression systems, also called gene switches, can be engineered to respond to any input, given that a sensor for this input exists. They are also at the heart of synthetic biology and are needed to construct more sophisticated regulatory circuits (also referred to as gene circuits), by for instance combining several gene switches with different inputs. To offer researchers interested in this technology the possibility to easily construct their own gene switches and circuits, we wrote an article for the book Mammalian Synthetic Promoters in the Methods in Molecular Biology book series, describing the foundational unit of the whole endeavor of synthetic biology: promoters for inducible gene expression.