Junk DNA May Act as a Molecular Bridge

Researchers have identified a cohesin-containing protein complex that reshapes chromatin to allow cohesins to bind to the repetitive Alu DNA sequences, areas of the genome not known to directly control genetic activity and sometimes referred to as junk DNA. Their finding was reported in the August 29, 2002, issue of Nature.

The investigators found that when the chromatin's histone proteins were methylated and acetylated, the chromatin structure relaxed to allow access to the DNA. However, if the Alu sequence on the DNA was itself methylated, then the cohesin could not bind to the DNA at that site.

"One thing that interested us is that there are 500,000-1,000,000 Alu repeats across the human genome,” explained Dr. Ramin Shiekhattar, an associate professor at The Wistar Institute (Philadelphia, PA, USA; www.wistar.upenn.edu) and the study's senior author. "These sequences are very common. And this makes sense if one of their roles is to bind to the bridging proteins, the cohesins, to keep the replicated DNA sisters together until it is time for them to separate. Multiple bridging sites throughout the DNA would be needed for this system to work. They could not be unique sequences.”

"The idea that a kind of code of modifications to the molecular packaging of DNA may govern gene activity is an intriguing one, said Dr. Shiekhattar. "If we were to better understand this code, it might provide us with important insights into diseases tied to problems in gene control, including developmental disorders and cancer.”



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