Beyond the fundamental DNA code, organisms also transmit chemical cues that instruct cells on gene utilization. This transmission, known as epigenetic inheritance, is particularly prevalent in plants. The implications of significant findings in this realm extend to agriculture, food security, and environmental preservation.
Esteemed researchers, Professors Rob Martienssen and Leemor Joshua-Tor from Cold Spring Harbor Laboratory (CSHL) and HHMI Investigators,
have undertaken pioneering research on how plants transfer the markers that maintain the dormancy of transposons, also known as jumping genes. When activated, transposons can disrupt other genes by relocating. Cells counteract this by adding regulatory markers to specific DNA sites through a process called methylation.
In their latest study, Martienssen and Joshua-Tor illuminate the role of the protein DDM1 in facilitating the enzyme responsible for placing these markers on new DNA strands. Plant cells require DDM1 because their DNA is tightly packaged. To ensure the orderly compactness of their genomes, cells wrap DNA around histones, proteins that aid in packaging. Martienssen explains that this process inadvertently obstructs DNA access for crucial enzymes. The histones must be maneuvered out of the way to initiate methylation.
DDM1, discovered by Martienssen and former CSHL colleague Eric Richards three decades ago, facilitates the sliding of DNA along histones to expose regions necessitating methylation. This movement is likened to a yo-yo gliding along a string, allowing histones to shift up and down DNA while maintaining attachment.
Genetic and biochemical experiments led Martienssen to identify the specific histones DDM1 displaces. Joshua-Tor employed cryo-electron microscopy to capture detailed images of the enzyme's interaction with DNA and associated packing proteins. The images elucidated how DDM1 binds to particular histones, leading to the remodeling of packaged DNA. Joshua-Tor noted that an unexpected bond discovered in this process correlated with the initial mutation found years ago.
The study further unveiled how DDM1's affinity for specific histones preserves epigenetic controls across generations. The team established that a histone unique to pollen is resistant to DDM1 and serves as a placeholder during cell division. Martienssen emphasizes that this histone retains the memory of its position during plant development, transferring this memory to the next generation.
While this study primarily delves into plants, its implications may extend to humans. Humans also rely on DDM1-like proteins to uphold DNA methylation. This discovery potentially unravels the mechanism by which these proteins maintain the functionality and integrity of our genomes.