Testing a proposed paradigm shift in analysis of phage DNA packaging

Area of Science:

• Molecular Biology
• Virology
• Biophysics

Background:

• Phage DNA packaging is crucial for viral replication and a target for antiviral strategies.
• Current models primarily focus on the ATPase-driven (type 1) packaging cycle.
• A potential secondary, shell expansion/contraction-driven (type 2) cycle has been hypothesized to resolve packaging stalls.

Purpose of the Study:

• To test a hypothesis proposing a dual-cycle mechanism for phage DNA packaging.
• To investigate the role of cellular macromolecular concentration on phage capsid behavior during packaging.

Main Methods:

• Utilizing a directed evolution-derived, 3-site mutant of phage T3.
• Adapting the phage mutant for propagation in concentrated bacterial cytoplasm.
• Observing packaging-active capsids under conditions of increased cellular macromolecular concentration.

Main Results:

• The study confirmed the prediction that increased cellular macromolecular concentrations induce hyper-expansion and contraction states in packaging-active capsids.
• Novel phage T3 capsid structures and behaviors, previously uncharacterized, were discovered.
• Evidence supports the existence of the proposed type 2 packaging cycle.

Conclusions:

• The findings support a paradigm shift in understanding phage DNA packaging, incorporating a dual-cycle mechanism.
• The proposed type 2 cycle likely functions as a backup to resolve stalls caused by non-DNA molecule packaging.
• This research opens new avenues for studying viral packaging dynamics and capsid evolution.