Binding of transcription factors to the clamp region of RNA polymerase. (A) Factor-free T. thermophilus elongation complex. (B) NusG-bound T. thermophilus elongation complex. (C) T. thermophilus RNA polymerase holoenzyme. Core RNA polymerase—green-cyan surface, NusG—purple–blue surface, σ (RpoD)—purple surface, nucleic acids—hot-pink cartoon.