Effect of supercoiling on the λ switch

Area of Science:

• Molecular Biology
• Epigenetics
• Bacteriophage Genetics

Background:

• The bacteriophage lambda (λ) switch maintains a stable lysogenic state but can rapidly transition to the lytic state upon DNA damage.
• The efficiency of this epigenetic switch is crucial for phage lifecycle regulation.

Purpose of the Study:

• To investigate the effect of DNA supercoiling on the efficiency of the λ epigenetic switch.
• To understand the role of supercoiling in maintaining the repressed state during induction.

Main Methods:

• Development of a novel assay using PNA-mediated tethering of a plasmid for single-molecule investigations.
• Analysis of the influence of supercoiling on CI-mediated DNA looping and epigenetic state transitions.

Main Results:

• Supercoiling significantly enhances the probability of CI-mediated DNA looping compared to non-supercoiled DNA.
• The presence of supercoils steepens the transition between looped and unlooped states, increasing the Hill coefficient.
• The transition occurs at the critical CI concentration required for pRM repression.

Conclusions:

• DNA supercoiling plays a critical role in maintaining the pRM-repressible state as CI concentration decreases during induction.
• Supercoiling prevents the autoregulation of the cI gene from interfering with the phage induction process.
• This mechanism contributes to the stability and responsiveness of the λ epigenetic switch.