Proteins transform DNA into 'molecul...( say for this protein's activity we tho...)

The researchers worked with a type of condensin found in the bacterium E. coli. Their experimental procedure consisted of attaching one end of a DNA molecule to a tiny plastic bead held by suction onto a micropipette. They then caused the DNA molecule to extend by flowing liquid past it, and exposed it to a solution containing the bacterial condensin protein. The researchers next added the energy-containing molecule ATP to the solution. After the ATP was added, they captured the other end of the condensin-treated DNA molecule with another plastic bead and proceeded to pull on the DNA with precisely measured force.

"We found that the DNA molecule had become much shorter in the presence of the condensin protein," said Bustamante. "And when we began to pull it apart carefully, we saw it extend in a sawtooth pattern of force, like the click-click-click of Velcro unzipping.

"When we pulled again a second time, much to our surprise, the process reproduced identically every tooth in the sawtooth pattern. We had never seen anything like that. We really thought that we were only seeing noise in the stretching of the DNA, but instead we were seeing a perfect registry in the sawtooth pattern," Bustamante said.

That perfect reproducibility strongly suggested to Bustamante and his colleagues that they were seeing a condensed structure with a well defined organization. "Every time we pulled it out and relaxed it, the molecule was able to return to the same initial or condensed form," said Bustamante. In fact, the researchers pulled and relaxed the same DNA molecule dozens of times, seeing the same sawtooth pattern of extension and condensation each time.

They also found that the energy-containing ATP molecule appeared to play a regulatory role, rather than providing energy for
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Contact: Jim Keeley
keeleyj@hhmi.org
301-215-8858
Howard Hughes Medical Institute
4-Jun-2004