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Figure 1.
Fig. 1. A Comparison of the structures of T7 RNAP initiation,
intermediate, and elongation complexes. The molecules have been
similarly oriented by superposition of their palm domains. The
C-terminal domain is shown as a surface with the thumb domain
removed to allow views of the DNA and RNA. The nontemplate
strand is shown in light green, the template in blue, and the
RNA transcript in red. Subdomains of the N-terminal domain are
colored yellow (C-helix), green (subdomain H), and purple (PBD).
(A) The initiation complex bound to the promoter has a 3-nt
transcript with its 5' end in contact with the PBD. (B) The
elongation complex exhibits a 220° right-hand rotation of
the PBD, a refolding of subdomain H onto the top of the
polymerase, and the formation of an elongated C-helix subdomain,
when compared to the initiation complex. (C) (Top) The T7 RNAP
in the 7-nt RNA intermediate complex is bound to both promoter
and downstream DNA. The PBD has rotated by 40° away from the
C-terminal domain, avoiding a steric clash with the transcript
and allowing for 7 bp of heteroduplex to form in the active
site. (Bottom) A schematic drawing of the sequences constituting
the subdomains of the N-terminal domain.
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