A new study published online in the journal eLife indicates that satellite DNA — commonly called ‘junk’ DNA — performs the vital function of ensuring that chromosomes bundle correctly inside the cell’s nucleus, which is necessary for cell survival; and this function appears to be conserved across many species.
Satellite DNA consists of a very simple, highly repetitive sequence of genetic code.
Although it accounts for a substantial portion of our genome, satellite DNA does not contain instructions for making any specific proteins. What’s more, its repetitive nature is thought to make the genome less stable and more susceptible to damage or disease.
Until fairly recently, scientists believed this ‘junk,’ or ‘selfish,’ DNA did not serve any real purpose.
“But we were not quite convinced by the idea that this is just genomic junk. If we don’t actively need it, and if not having it would give us an advantage, then evolution probably would have gotten rid of it. But that hasn’t happened,” said lead author Professor Yukiko Yamashita, from the University of Michigan Life Sciences Institute.
Professor Yamashita and co-authors decided to see what would happen if cells could not use this pericentromeric satellite DNA.
Because it exists in long, repetitive sequences, they could not simply mutate or cut the entire satellite DNA out of the genome. Instead, they approached the question through D1, a protein known to bind to satellite DNA.
The scientists removed D1 from the cells of fruit flies (Drosophila melanogaster), a commonly used model organism.
And they quickly noticed that germ cells — the cells that ultimately develop into sperm or eggs — were dying.
Further analysis revealed that the dying cells were forming micro-nuclei, or tiny buds, outside the nucleus that included pieces of the genome. Without the entire genome encapsulated in the nucleus, the cells could not survive.
“The D1 protein binds onto the satellite DNA to pull all of the chromosomes together in the nucleus,” the study authors explained.
“If the D1 protein cannot grab the satellite DNA, the cell loses its ability to form a complete nucleus and ultimately dies.”
“The protein has multiple binding sites, so it can bind onto multiple chromosomes and package them together in one place, preventing individual chromosomes from floating out of the nucleus,” Professor Yamashita added.
The researchers conducted similar experiments using mouse cells and found the same results. When they removed a protein that normally binds to mouse satellite DNA, the cells again formed micro-nuclei and did not survive.
The similar findings from both fruit fly and mouse cells lead the team to believe that satellite DNA is essential for cellular survival, not just in model organisms, but across species that embed DNA into the nucleus — including humans.
Madhav Jagannathan et al. 2018. A conserved function for pericentromeric satellite DNA. eLife 7: e34122; doi: 10.7554/eLife.34122