DNA-based Molecular Motors

Figure 2    RNA polymerase:
a linear tracking motor

The majority of DNA-based molecular motors are ‘linear-tracking’ motors (they can be likened to railway trains running along railway tracks); they use the repetitive nature of the DNA sequence to enable them to move along DNA.  The best example, and one of the most closely studied at the single-molecule level, is RNA polymerase (Harada et al., 1999; Schafer et al., 1991; Wang et al., 1998) .  This enzyme is responsible for synthesis of messenger RNA (the ‘reading’ intermediate between DNA and protein) and uses the energy of this synthesis reaction to enable movement along the DNA, reading the bases as it moves and copying them into a single chain of RNA.  Single-molecule studies of this motor have made use of optical tweezers to hold a polystyrene bead attached to the DNA, while the motor is attached to a surface (Figure 2).  Many other motors follow this pattern of linear-tracking movement but, generally, have different functions and as a consequence interact differently with DNA (e.g. DNA helicases are responsible for unwinding the two strands of DNA, DNA polymerases synthesis a new strand of DNA and DNA repair enzymes are able to detect and remove damaged bases in the DNA).  However, Type I R-M enzymes are unusual as molecular motors, as the enzyme binds to a specific site on the DNA and remains bound at that site while moving the adjacent DNA in the manner of a nano-actuator (Figure 3, 4 & 5).  They can be likened to the spool of a fishing rod, where the fishing line is DNA.

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