Understanding resource competition to achieve predictable synthetic gene expression in eukaryotes

Synthetic gene expression in engineered cells typically depends on the host cell’s limited pool of intracellular resources, which can lead to resource competition between native and engineered functions and may cause unanticipated effects in the host and synthetic gene circuit. For example, competition for shared resources may lead to cellular overload affecting physiological functions (that is, cellular burden) and negatively affect synthetic construct performance to the point of unreliability. This fundamental problem has implications for the use of synthetic genetic constructs in cell engineering for foundational research, therapy and bioprocessing. Resource competition has mainly been investigated in model bacteria systems, but also considerably affects eukaryotic systems, including mammalian cells. In this Review, we discuss resource competition beyond bacteria, outlining how it can lead to gene expression coupling, gene expression and metabolic burden. We also examine ways to quantify cellular burden in mammalian cells, and investigate circuit-centric and host-centric mitigation strategies, highlighting important implications of resource competition for cell engineering in therapeutic and bioproduction applications as well as in fundamental biology studies. Cell engineering by synthetic biology typically relies on synthetic gene constructs that compete with the host cell for intracellular resources. This Review discusses how such resource competition can impact mammalian cell engineering and outlines strategies for how to mitigate cellular burden using circuit-centric and host-centric approaches.