Histone H3K9 methylation and Heterochromatin Protein 1 do not limit DNA accessibility in living S. pombe cells

Inside every living cell, the genetic code is not floating freely but is tightly wrapped around protein spools, forming a complex material called chromatin. This packaging serves a dual purpose: it protects the long strands of DNA and controls which parts of the genetic instruction manual can be read at any given moment. For decades, scientists have operated under the assumption that this wrapping acts as a gatekeeper, specifically in regions known as heterochromatin. These are the tightly packed sections of the genome, often marked by chemical tags on the protein spools and bound by specific proteins, which were thought to lock the DNA away so tightly that the cellular machinery could not reach it. This idea of a locked-down region has been central to understanding how cells regulate their genes, suggesting that the primary job of these chemical marks and binding proteins is to physically block access to the DNA.

Researchers have long studied this phenomenon in various organisms, including humans and budding yeast, where evidence from isolated cell nuclei supported the view that heterochromatin is indeed a barrier to entry. However, a team of scientists turned their attention to the fission yeast, a single-celled organism that serves as a powerful model for studying these tightly packed regions. In this yeast, the heterochromatin is defined by specific chemical modifications on its histone proteins and the presence of a binding protein known as HP1. The prevailing theory held that these features worked together to seal off the DNA, preventing it from being accessed. To test whether this model held true in a living, breathing cell rather than a static sample, the researchers developed a new method to probe the accessibility of DNA directly inside the organism. They introduced a system that could activate a specific enzyme to modify the DNA, allowing them to measure how easily this enzyme could reach different parts of the genome in real time.

The results of this experiment challenged the long-held belief that the chemical marks and binding proteins act as a physical wall. When the scientists measured the DNA in living fission yeast cells, they found that the tightly packed heterochromatin was just as open and accessible as the more loosely packed regions of the genome. Even the centromeres, the specialized areas where chromosomes attach to the cell's internal skeleton, were fully accessible in this organism. This stands in contrast to findings in other species, where centromeres are often inaccessible. The study suggests that in living fission yeast, the chemical tags and the binding proteins do not function by blocking the door to the DNA. Instead, the genome appears to be globally dynamic, with the machinery able to reach almost any part of the DNA regardless of the local packaging style.

The story changes, however, when the researchers looked at the same cells after they had been broken open to isolate the nuclei. In these isolated samples, the chromatin became largely inaccessible, behaving exactly as the traditional model predicted. The researchers determined that this shift was not caused by the loss of the chemical marks or the binding proteins, but rather by the physical structure of the DNA itself. In the isolated nuclei, the protein spools were packed so tightly together that there was almost no space between them for enzymes to enter. This tight spacing, which occurs when the cell is no longer intact, creates a physical barrier that does not exist in the living cell. Consequently, the study indicates that the chemical system of marks and binding proteins in this yeast does not repress gene activity by preventing access to the DNA. The idea that these proteins act as a lock to keep the DNA hidden appears to be a misinterpretation of what happens when cells are removed from their natural, living state.