Fig. 12.15.

T7 RNA polymerase initiation and elongation complexes. (a) Initiation complex containing the promoter dsDNA and a three-nucleotide ssRNA transcript. The sequences of the nucleic acids are shown with the portions visible in the crystal structure shaded in gray (top). RNA synthesis starts at the +1 position. Polymerase is shown in the absence and presence of the bound nucleotides for clarity (bottom). The N-terminal domain, fingers, palm, and thumb domains are shown in yellow, blue, green, and red, respectively. The specificity loop in the fingers domain is shown in purple. The template (coding) strand, nontemplate strand, and the RNA transcript are shown in magenta, orange, and blue ribbons, respectively. (b) Elongation complex containing dsDNA and a ten-nucleotide RNA transcript. Note three nucleotides at the 5′-end of the RNA transcript are separated from the template and are exiting the active site