Systems for controlling gene expression in mammalian cells have a wide range of applications in medicine, biotechnology and basic science. An ideal gene regulatory system would allow for precise and specific control over the magnitude and kinetics of gene expression in space and time, while also exerting minimal influence on other genes and cellular components. Several gene regulatory systems have been developed in which orthogonal transcription machinery from prokaryotes or insects has been imported into mammalian cells and used to control the expression of a specific gene. Despite the transformative impact of these systems in biomedical and biological research, several limitations of these technologies restrict the scope of possible applications. For example, gene expression in these systems is controlled by a freely diffusible small molecule, such as an antibiotic or steroid. Consequently, it is not possible to achieve spatial control over gene expression within cell culture, tissues, or whole organisms. This is in contrast to natural mechanisms of biological regulation in which spatial control is critical, such as developmental patterning and tissue morphogenesis. Second, dynamic gene regulation requires the removal of these small molecules, which may be slow, laborious, and/or impractical for a particular application. To overcome these limitations, we have engineered an optogenetic system in which the magnitude of gene expression in human cells can be finely tuned by photoregulated synthetic transcription factors.

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