FIGURE 1.

Structure of T7 gp4 and location of the A257T mutation. A, T7 gp4 hexameric ring showing the arrangement of the helicase and primase domains (Protein Data Base ID code 1Q57). B, One subunit of the gp4 ring showing the 26-amino acid linker region between the C-terminal helicase and N-terminal primase domains. Ala²⁵⁷ is present in the linker region. C, close-up view of the Ala²⁵⁷ residue in relationship to proximal Gln²⁵³ and Val²³⁰. D, close-up view of A257T mutation showing clashes of Thr²⁵⁷ with the backbones of residues Gln²⁵³ and Val²³⁰.