[Visualization of elongation complexes for t7 Rna polymerase by atomic force microscopy]

Complexes of bacteriophage T7 RNA polymerase (RNAP) with a DNA template for transcription elongation were visualized by atomic force microscopy (AFM). Fragment of pGEMEX linear DNA with length of 1414 bp carrying promot- er and terminator of bacteriophage T7 was DNA template for transcription. Promoter and terminator are located unsymmetrically on the ends of DNA template. Images of stable complexes of T7 RNAP with terminal fragments of DNA template were obtained for single molecules. Complexes of a single DNA template molecule with 2-3 T7 RNAP molecules corresponding to stages of initiation, elongation and termination of transcription were visualized under the elimination of non-specific DNA-protein binding. Also complexes of DNA, RNAP and RNA transcripts were imaged. Our results suggest that the initial stage is the formation of complex between T7 RNAP and terminal fragment of DNA template. Because promoter is localized near DNA terminus, it makes impossible an ommision of the promoter site.