A new study from Umeå University is shaking up 20+ years of thinking in biology. Scientists have long believed a specific chemical change on proteins in our cells acts as the main way cells “remember” what they are (like a skin cell staying a skin cell). But this research using fruit flies shows that idea might be wrong.
Cells in your body divide many times, but a liver cell doesn’t suddenly become a brain cell. This “memory” comes from epigenetics chemical tags that keep certain genes turned off forever. The Polycomb system is like a team of proteins that locks these genes down so cells stay true to their type.
Why Has Everyone Thought H2A Modification Was the Key to Cell Memory?
For over 20 years, experts believed the main lock was a tiny chemical addition called ubiquitination on histone H2A (a protein that packages DNA). This mark, added by complexes like PRC1, was seen as the universal signal that says: “Keep this gene silent during cell division!”
How Did Fruit Flies Help Scientists Test This Long-Held Idea?
Fruit flies (Drosophila melanogaster) have a simpler setup than humans only three versions of a key protein (PCGF) instead of six. Researchers focused on one called Siesta (the fly version of human PCGF3). Siesta adds most of the H2A marks across the entire fly genome.
(Adult fruit fly — the simple model organism that made this discovery possible.)
What Surprising Result Did They Find About Gene Silencing?
Even without Siesta (and without most H2A marks), the flies still correctly silenced important developmental genes. The “memory” worked perfectly — no mix-ups in gene patterns during development. This means H2A ubiquitination is not the essential memory mechanism everyone thought it was.
Why Did Mutant Flies Move Slowly And What Does That Mean?
Siesta mutants had a completely unexpected problem: their larvae crawled slowly and irregularly. This movement issue had nothing to do with gene silencing or H2A marks. It suggests Siesta does something else entirely perhaps in nerves or muscles outside the normal Polycomb “memory” role.
Is It Time to Change How We Label These Protein Complexes?
Yes, the study says grouping all RING1-based complexes as “PRC1 variants” is misleading. Siesta complexes aren’t part of the classic Polycomb repression machinery. Researchers call for an update to this classification to reflect the real biology.
(Diagram showing how Polycomb complexes add the H2A ubiquitination mark — now shown not to be the core repression tool.)
What Bigger Questions Does This Open Up for Biology?
If H2A isn’t the main memory lock, what other targets or tricks do Polycomb proteins use to keep genes off? This work gives new tools (like fly mutants) to find out and could rewrite parts of how we understand cell development, diseases like cancer (where Polycomb often goes wrong), and aging.
The study was published in Science Advances (March 2026): Kahn TG, et al. “Polycomb repression works without Siesta, the Drosophila ortholog of mammalian PCGF3.” DOI: 10.1126/sciadv.aec0733
