Structure of DNA within three isometric bacteriophages

Area of Science:

• Structural biology
• Molecular virology
• Biophysics

Background:

• Bacteriophages are viruses that infect bacteria.
• Understanding bacteriophage structure is crucial for virology and genetic engineering.
• Previous studies utilized electron microscopy and birefringence, but DNA's precise arrangement remained unclear.

Purpose of the Study:

• To model the three-dimensional structure of DNA within bacteriophages T7, P22, and phiCd-1.
• To investigate the DNA coiling and orientation within the phage head.
• To determine the relationship between DNA structure and phage morphology.

Main Methods:

• Transient electric dichroism measurements on intact bacteriophages.
• Analysis of reduced dichroism at perfect orientation.
• Calculation of permanent dipole moments and equivalent sphere radii.
• Circular dichroism spectroscopy to assess local DNA helicity.

Main Results:

• DNA is organized into closely packed, co-axial solenoids within the phage head.
• The solenoid axis is tipped 43.5° ± 2.5° relative to the phage orientation axis.
• Large permanent dipole moments were observed for T7, phiCd-1, and P22.
• Calculated radii align with electron microscopy and birefringence data.
• Local DNA helicity is consistent across all three bacteriophages.

Conclusions:

• The study provides a detailed model for DNA packaging in bacteriophages T7, P22, and phiCd-1.
• The findings reveal a specific solenoid structure for DNA organization.
• This structural insight contributes to understanding viral assembly and genome packing.