Molecular gymnastics: distortion of an RNA polymerase sigma factor.

A recent structure obtained by nuclear magnetic resonance (NMR) spectroscopy shows that the binding of a small phage factor to the sigma(70) subunit of Escherichia coli RNA polymerase induces an unprecedented remodeling of a region of sigma(70), converting a DNA-binding helix-turn-helix into a continuous pseudohelix. This conformational change suggests how the phage factor can function both as an inhibitor and co-activator of transcription.