In follow-up work, Ha and his team brought out another tool, a nano-sized “tweezer,” based on the phenomenon of optical trapping, in which a beam of bright light effectively sucks a tiny object toward its center. Using a laser, they optically trapped a microsphere that was already linked to FRET-tagged DNA; the DNA's other end was bound to a molecular anchor. One of Ha's grad students, Ruobo Zhou, used the technique to gradually pull the single-stranded DNA away from SSB while recording the applied tension as well as FRET signals. The SSB protein still seemed able to move back and forth, even when partly unwrapped.

To Ha, these findings indicate that repair and maintenance proteins can move along a single-stranded nucleotide structure much more easily and robustly than had been thought. RecA's apparent prodding of SSB so that it keeps moving in a single direction also suggests to him “a general mechanism for displacement of SSB.”

Ha and his lab group are trying to refine their measurement techniques to be able to track dimensions smaller than a single DNA nucleotide. “That's really the ultimate resolution you can ask for,” he says. But much of Ha's time these days is also taken up with the job of systematizing and disseminating these new and still somewhat arcane research tools. “Eventually I want these techniques to be used by every biologist.”

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