Machinations of a maxwellian demon.

The mechanisms by which RNA polymerase moves along DNA during elongation have been difficult to determine experimentally. In this issue of Cell, Bar-Nahum et al. (2005) show that back and forth sliding of RNA polymerase on DNA may be coupled to bending of an alpha helix, which can alternately occlude and expose the NTP binding site. Transcription factors can regulate elongation by modulating this bending motion.