The MYC oncogene contributes to an estimated 100,000 cancer-related deaths annually in the United States and is associated with aggressive tumor progression and poor clinical outcome. MYC is a nuclear transcription factor that regulates a myriad of cellular activities and has direct interactions with hundreds of proteins, which has made a unified understanding of its function historically difficult.

In recent years, several groups have put forth a new hypothesis that questions the prevailing view of MYC as a gene-specific transcription factor and instead envision it as a global amplifier of gene expression. Instead of being an on/off switch for transcription, MYC is proposed to act as a `volume knob' to amplify and sustain the active gene expression program in a cell. The scope of the amplifier model remains controversial in part because studies of MYC largely consist of cell population-based measurements obtained at fixed timepoints, which makes distinguishing direct from indirect consequences on gene expression difficult. A high-temporal, high-spatial precision viewpoint of how MYC acts in single living cells does not exist.

To evaluate the competing hypotheses of MYC function, we developed a single-cell assay for precisely controlling MYC and interrogating the effects on transcription in living cells. We engineered `Pi-MYC,' an optogenetic variant of MYC that is biologically active, can be visualized under the microscope, and can be controlled with light. We combined Pi-MYC with single-molecule imaging methods to obtain the first real-time observations of how MYC affects RNA production and transcription factor mobility in single cells. We show that MYC increases the duration of active periods of genes population-wide, and globally affects the binding dynamics of core transcription factors involved in RNA Polymerase II transcription complex assembly and productive elongation. These findings provide living, single-cell evidence of MYC as a global amplifier of gene expression, and suggests the mechanism is by stabilizing the active period of a gene through interactions with core transcription machinery.