Direct demonstration and quantification of long-range DNA looping by the lambda bacteriophage repressor

Area of Science:

• Molecular Biology
• Genetics
• Biophysics

Background:

• The lambda repressor (CI protein) plays a crucial role in bacteriophage lambda's life cycle, regulating lysogeny and lysis.
• DNA looping mediated by CI protein has been hypothesized to strengthen repression and regulate switching between lysogenic and lytic pathways.

Purpose of the Study:

• To investigate the role of DNA looping by the lambda repressor (CI protein) in gene regulation.
• To quantitatively analyze dynamic CI-mediated DNA looping using tethered particle motion experiments.

Main Methods:

• Tethered particle motion experiments were conducted on single DNA molecules.
• Analysis involved comparing DNA with intact operators versus mutated o3 operators.
• Thermodynamic modeling was used to determine the free energy of loop formation.

Main Results:

• CI octamer-mediated loop formation has a free energy of 1.7 kcal/mol.
• Loop formation free energy decreased to -0.7 kcal/mol when a tetramer was added (octamer+tetramer-mediated loop).
• The o3 operator sites were found to be critical for stable CI-mediated DNA loops.

Conclusions:

• Stable and spontaneous CI-mediated DNA loops require augmentation by a tetramer of CI bound at o3 sites, in addition to an octamer at other operators.
• The o3 sites are essential for CI protein-secured loops that attenuate cI expression.
• These findings support the hypothesis that DNA looping by CI protein is integral to regulating bacteriophage lambda's life cycle.