EVIDENCE OF DNA BENDING IN TRANSCRIPTION COMPLEXES IMAGED BY SCANNINGFORCE MICROSCOPY

Complexes of Escherichia coli RNA polymerase with DNA containing the l ambda P(L) promoter have been deposited on mica and imaged in air with a scanning force microscope. The topographic images reveal the gross spatial relations of the polymerase relative to the DNA template. The DNA appears bent in open promoter complexes containing RNA polymerase bound to the promoter and appears more severely bent in elongation com plexes in which RNA polymerase has synthesized a 15-nucleotide transcr ipt. This difference could be related to the conformational changes th at accompany the maturation of open promoter complexes into elongation complexes and suggests that formation of the elongation complex invol ves a considerable modification of the spatial relations between the p olymerase and the DNA template.