FIGURE 8.

DNA unwinding by hexameric gp4. The schematic depicts the unwinding of a replication fork by the T7 helicase/primase. During DNA unwinding, gp4 interacts with 1 ssDNA at the center of the ring with the help of residues at central β-hairpins, whereas excluding the complementary strand by Phe⁵²³ from the outer surface. The inset shows the location of central β-hairpins (blue) and subunit-interface β-hairpins (red) in the crystal structure of the helicase domain of T7 gp4 (PDB code 1E0J) drawn on the surface view by PyMOL. The schematic also shows the location of these β-hairpins in the hexameric helicase while unwinding a replication fork. The orientation of Phe⁵²³ (cyan) toward the outer surface can be seen only from the interfaces with the empty state of NTP-binding site. Phe⁵²³ in other sites are buried in the structure and are not visible in the outer surface.