ITHACA, N.Y. With an eye toward understanding DNA replication, Cornell researchers have learned how a helicase enzyme works to actually unzip the two strands of DNA. (Nature, online Sept. 18, 2011.)
At the heart of many metabolic processes, including DNA replication, are enzymes called helicases. Acting like motors, these proteins travel along one side of double-stranded DNA, prompting the strands to "zip" apart.
What had been a mystery was the exact mechanics of this vital biological process how individual helicase subunits coordinate and physically cause the unzipping mechanism.
Cornell researchers led by Michelle Wang, professor of physics and an investigator of the Howard Hughes Medical Institute (HHMI), have observed these processes by manipulating single DNA molecules to watch what happens when helicases encounter them, and how different nucleotides that fuel the reactions affect the process. For their experiments they used an E. coli T7 phage helicase, a type with six distinct subunits, which is a good representation of how many helicases work.
"This is a great demonstration of the power of single-molecule studies," said Wang, whose lab specializes in a technique called optical trapping. To record data from single molecules, the scientists use a focused beam of light to "trap" microspheres attached to the molecules.
Prior to this work, researchers from other labs had found that the nucleotide dTTP (deoxythymidine triphosphate) was a "preferred" fuel for the helicase, and that the helicase apparently wouldn't unzip DNA if ATP (adenosine triphosphate) was provided as fuel. Wang and her colleagues found this puzzling, because ATP is known to be the primary fuel molecule in living organisms.
In their latest work, they discovered that, in fact, ATP does cause unwinding, but only in the single-molecule study could they confirm this. In normal biochemical studies, ATP doesn't seem to work, because it causes helicas
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