Figure 5.

Strand displacement DNA synthesis. Strand displacement DNA synthesis was measured by using a 70-nt circular dsDNA template with a 40-nt 5′-oligo(dT) DNA tail (as shown in Insets). The 5′ ssDNA tail allowed the gene 4 protein to assemble on the ssDNA. The hexameric gene 4 protein translocates 5′ to 3′ along the ssDNA direction until it encounters the 70-nt circular duplex DNA region. The helicase activity of gene 4 protein unwinds the duplex region and allows T7 DNA polymerase to carry out DNA synthesis on the circular ssDNA template. The reaction was performed as described in Materials and Methods. The amount of [α-³²P]dGMP incorporated into the DNA by the wild-type T7 DNA polymerase (○) or T7 polΔ401–404 (●) is plotted vs. increasing polymerase concentrations either in the presence of the 63-kDa gene 4 protein (A) or the 56-kDa gene 4 protein (B).