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Figure 1.
Figure 1. Crystallographic Structure of the T7
Primase-Helicase(A) A ribbon diagram of T7 primase-helicase
shows that the helicase domains (red) and associated DNA binding
loops (yellow) are arranged in a ring with approximate 7-fold
rotational symmetry. This view shows the C-terminal side of the
ring.(B) A view of the N-terminal side of the ring shows seven
primase domains (blue) are loosely arrayed in different
orientations on top of the helicase ring (red). The active sites
of the primases (gold balls) roughly face the circumference of
the ring and point toward the neighboring subunit. One of the
primase domains, designated with an orange ball, tilts toward
the center of the ring where its active site is positioned for
interaction with DNA passing through the central channel.(C) A
schematic diagram, oriented as in (B), shows the relative
orientations of the primase domains (blue balls with N-terminal
“hats”) and their active sites (light blue dots). The inward
facing primase domain has an orange hat. The underlying helicase
domains (red) are shown with yellow arrows that point toward the
helicase active site. Ribbon diagrams were created with the
program RIBBONS (Carson, 1997) and rendered with POV-Ray
(Amundsen et al., 2000).
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