Dual functions, clamp opening and primer-template recognition, define a key clamp loader subunit

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Clamp loader proteins are essential for DNA replication, facilitating the assembly of sliding clamps onto primer-template DNA.
• The specific sites responsible for primer-template DNA binding and selection by clamp loaders have remained largely unknown.
• Clamp loaders are multi-protein complexes with varying subunit compositions across different organisms.

Purpose of the Study:

• To identify the specific subunit and residues within the *E. coli* gamma complex clamp loader responsible for primer-template DNA recognition.
• To elucidate the mechanism by which clamp loaders bind and select their DNA targets for clamp assembly.

Main Methods:

• UV-induced protein-DNA cross-linking coupled with mass spectrometry was employed to analyze protein-DNA interactions.
• The study focused on the *Escherichia coli* gamma complex clamp loader and its interaction with primer-template DNA.

Main Results:

• The delta subunit of the *E. coli* gamma complex was found to be in close contact with the primer-template DNA junction.
• Tryptophan 279 within the delta subunit's C-terminal domain was identified as potentially playing a key role in primer-template recognition, located near the 3'-OH primer end.
• The delta subunit exhibits dual functionality, binding both the beta clamp and primer-template DNA, with sites positioned for efficient DNA entry and template strand exit.

Conclusions:

• The delta subunit is a key component for primer-template DNA binding and recognition in the *E. coli* gamma complex clamp loader.
• These findings suggest a conserved mechanism for DNA target selection in clamp loaders across different organisms, potentially including *S. cerevisiae* RFC complex.
• Understanding these interactions provides insights into the regulation and processivity of DNA replication.