Small terminase couples viral DNA binding to genome-packaging ATPase activity

Area of Science:

• Virology
• Structural Biology
• Molecular Biology

Background:

• Viral genome packaging relies on DNA-packaging motors.
• These motors typically consist of large (L) and small (S) terminase subunits and portal proteins.

Purpose of the Study:

• To elucidate the structure of the bacteriophage P22 small terminase.
• To understand the mechanism of viral DNA packaging and terminase activation.

Main Methods:

• X-ray crystallography (1.75 Å resolution) to determine the S-terminase structure.
• Biochemical assays to assess DNA binding and ATPase activity.

Main Results:

• The 1.75 Å crystal structure of bacteriophage P22 S-terminase in a nonameric conformation was determined.
• A central channel approximately 23 Å in diameter, suitable for hydrated B-DNA, was observed.
• The C-terminal 23 residues of S-terminase are crucial for DNA binding and L-terminase assembly.
• DNA binding to S-terminase specifically activates L-terminase ATPase activity.

Conclusions:

• The structure provides insights into the DNA-packaging motor's architecture.
• DNA-dependent activation of L-terminase ensures specific packaging of viral DNA.
• This mechanism prevents packaging of host DNA and wasteful ATP hydrolysis, maintaining packaging fidelity.