Strand exchange through a DNA-protein complex requires a DNA helicase

Area of Science:

• Molecular Biology
• Genetics
• Virology

Background:

• Homologous strand exchange is crucial for genetic recombination.
• Phage T4 uvsX and gene 32 proteins facilitate in vitro DNA strand exchange with naked DNA.
• Strand exchange is hindered by sequence-specific DNA-protein complexes, posing a challenge for in vivo recombination.

Purpose of the Study:

• To investigate the factors required for homologous strand exchange on protein-complexed DNA substrates.
• To determine if additional factors are necessary for recombination when DNA is bound by proteins in vitro.
• To elucidate the role of DNA helicases in overcoming DNA-protein complex barriers during recombination.

Main Methods:

• In vitro assays using purified phage T4 recombination proteins (uvsX and gene 32).
• Employing protein-bound DNA substrates, specifically a RNA polymerase-promoter complex.
• Introducing the T4 dda DNA helicase to assess its effect on branch migration through the complex.

Main Results:

• Phage T4 uvsX and gene 32 proteins alone could not mediate strand exchange through a protein-bound DNA complex.
• The addition of the T4 dda DNA helicase enabled the phage recombination machinery to drive branch migration through a RNA polymerase-promoter complex.
• This study presents the first successful in vitro demonstration of homologous strand exchange using protein-bound DNA substrates.

Conclusions:

• A DNA helicase, such as the T4 dda protein, is a necessary component for mediating homologous strand exchange on protein-bound DNA.
• DNA helicases likely play a critical role in the in vivo recombination machinery by overcoming DNA-protein complex impediments.
• These findings reveal a key mechanism by which bacteriophages achieve genetic recombination in the complex cellular environment.