Discontinuous mechanism of transcription elongation.

During transcription elongation, three flexibly connected parts of RNA polymerase of Escherichia coli advance along the template so that the front-end domain is followed by the catalytic site which in turn is followed by the RNA product binding site. The advancing enzyme was found to maintain the same conformation throughout extended segments of the transcribed region. However, when the polymerase traveled across certain DNA sites that seemed to briefly anchor the front-end domain, cyclic shifting of the three parts, accompanied by buildup and relief of internal strain, was observed. Thus, elongation proceeded in alternating laps of monotonous and inchworm-like movement with the flexible RNA polymerase configuration being subject to direct sequence control.