The process of transferring genetic memory to plants has been identified. This was discovered by scientists at the Cold Spring Harbor Laboratory and researchers at the Howard Hughes Medical Institute, Rob Martinsen and Lemur Joshua Tur, in a study published in the journal Cell. When organisms pass on their genes to future generations, they don’t just pass on their genes to future generations, they include the code written in their DNA, some also pass on chemical markers that instruct cells how to use that code. The transmission of these signs to future generations is known as epigenetic inheritance, and it is particularly common in plants. These findings could have future implications for agriculture, food supplies and the environment. The researchers studied how plants transmit the markers that keep transposons inactive, also known as transposons. When activated, it can move around and disable other genes. To silence them and protect the genome, cells add regulatory markers to specific sites in the DNA. This process is called methylation. Martinsen and Joshua Tor have now shown how the DDM1 protein makes way for the enzyme that marks these new strands of DNA. Plant cells need DDM1 because their DNA is tightly packed. To keep their genomes compact and orderly, cells wrap their DNA around packaging proteins called histones. “But this blocks access to the DNA for all kinds of important enzymes,” Martinsen explained. “Before methylation can take place,” Martinsen continued, “the histones must be removed or slid away.” DDM1 was first discovered by Martinsen and his former CSHL colleague Eric Richards thirty years ago, and found that it slides DNA along its packaging proteins to reveal methylation sites. Martinsen likens the motion to the motion of a yo-yo sliding along a string.”
“Histones can move up and down the DNA, exposing pieces of DNA at a time, but they never fall off,” Martinsen said. Through genetic and biochemical experiments, Martensen identified the exact histones that DDM1 displaced. Joshua Torr used cryogenic electron microscopy to take detailed images of the enzyme interacting with DNA and its associated packaging proteins. The team of scientists was able to see how DDM1 attaches to specific histones to remodel the packaged DNA. “It turns out that the unexpected link linking DDM1 corresponds to the first mutation found all those years ago,” said Joshua Tor.
Experiments also revealed how DDM1’s affinity for certain histones maintains epigenetic controls across generations. The work has documented that only histones in pollen are resistant to DDM1 and act as a placeholder during cell division. “Histones remember where the histones were during plant development and retain this memory in the next generation,” Martinsen noted. Plants may not be alone. Humans also rely on DDM1-like proteins to maintain DNA methylation. Thus, the new discovery may help explain how these proteins keep our genomes functional and intact. Favorite
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