RNAPII and XPC remodel the 3D genome for UV repair

Inside the nucleus of every human cell, the DNA that carries our genetic instructions is not a loose, tangled string. Instead, it is folded into a complex, three-dimensional architecture, organized into loops and domains that bring distant parts of the genome close together. This folding is not static; it is highly elastic, constantly shifting to allow the cell to read its genes or to respond to damage. One of the most dangerous threats to this genetic code is ultraviolet light, which can cause chemical lesions in the DNA. To survive, cells rely on a repair system called nucleotide excision repair, a mechanism that scans the genome, finds these damaged spots, and fixes them. For decades, scientists understood how this repair machinery worked at the molecular level, but a crucial question remained: how does this repair process interact with the large-scale, three-dimensional structure of the genome? Does the damage simply sit within a static structure, or does the act of repairing it actively reshape the space around it?

A new study addresses this question by looking at what happens when human cells are exposed to ultraviolet light and how their internal architecture responds. Previous research had shown that UV light causes the genome to restructure, but those studies were limited to cells that could repair DNA normally. This made it difficult to tell whether the changes in structure were a direct result of the repair machinery working, or simply a side effect of the damage itself. To solve this, researchers examined cells that were specifically deficient in repair, comparing them to healthy cells, and combined these observations with computer simulations of how DNA loops behave. They focused on two key proteins: RNA polymerase II, a machine that reads genes to make proteins, and XPC, a protein that acts as a sensor for DNA damage across the entire genome.

The researchers found that when UV light hits the cell, it triggers a specific chain of events that tightens the genome's structure. Normally, the exposure to UV light causes the cell to shut down its transcription, a process that would typically cause the loops of DNA to lengthen and the genome to loosen. However, the study reveals that the repair machinery fights against this loosening. Both RNA polymerase II and the XPC protein act as anchors that constrain the movement of DNA loops. In computer simulations that modeled how DNA is pulled and folded, these proteins were shown to stop the loops from stretching out. Instead of the genome becoming more open and disordered, the presence of these proteins leads to shorter chromatin loops and stronger, more reinforced domains.

This structural tightening is not a passive accident; it appears to be an active strategy to help the cell survive. By keeping the loops short and the domains reinforced, the cell creates a more compact environment that makes it easier for the repair machinery to find and fix the damaged spots. The study suggests that RNA polymerase II does more than just start the process of fixing damage in specific genes; it helps create a genome-wide state that is ready for repair. Similarly, XPC is shown to be an active driver of this three-dimensional reorganization, rather than just a passive observer of the damage. The findings indicate that the repair process and the genome's structure are deeply coordinated, with the repair machinery actively reshaping the nuclear landscape to ensure the integrity of the genetic code is preserved.