The best way to obtain unambiguous information about the function of biomolecules is to study their function at the single molecule level. By attaching a small tag such as a single fluorescent dye molecule to biomolecules, or a tag that is huge compared to the size of a biomolecule, we have been able to image the individual behaviours in real time under an optical microscope. Huge tags such as micrometer-sized plastic beads also allow the manipulation of individual molecules with, e.g. optical or magnetic tweezers. Using these techniques, we have observed individual behaviours of RNA polymerase, a molecular machine that copies the genetic information on DNA onto a messenger RNA. We found that some of the RNA polymerase molecules underwent linear diffusion along DNA, helping RNA polymerase to search for the promoter. We also found that a single RNA polymerase molecule rotated around the right-handed screw axis of the double helix of DNA during transcription with a rotary torque of >5?pN?nm. The present methods are potentially applicable to the examination of a wide variety of protein–nucleic acid interactions, especially those involved in the process of transcription.
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