Protamine folds DNA into flowers and loop stacks.

Area of Science:

• Molecular Biology
• Biophysics
• Chromatin Structure

Background:

• Sperm DNA achieves extreme compaction through protamine proteins.
• Protamine-DNA interaction involves electrostatic coating and folding into toroids.
• The precise mechanism of toroid formation remains largely unknown.

Purpose of the Study:

• To investigate the initial stages of DNA toroid formation.
• To study the folding of intermediate DNA lengths (639-3003 bp) by protamines.
• To identify structural intermediates in the protamine-mediated DNA folding pathway.

Main Methods:

• Observation of DNA folding structures at varying protamine concentrations (0.2 μM to ≥2 μM).
• Analysis of DNA lengths ranging from 2 to 10 loops.
• Characterization of intermediate structures like loops, flowers, and loop stacks.

Main Results:

• At low protamine concentrations (~0.2 μM), DNA forms 'flowers' through multiple small bending steps, condensing DNA to 25% of its length in seconds.
• At higher protamine concentrations (≥2 μM), DNA folds into 'loop stacks' with vertically arranged loops.
• Two novel structures, flowers and loop stacks, are identified as potential early intermediates in toroid formation.

Conclusions:

• A two-step mechanism for DNA folding at this length scale is proposed: 1) protamine binding and bending into loops and flowers, and 2) flower collapse into loop stacks.
• Protamine utilizes a 'bind-and-bend' mechanism for rapid DNA folding.
• This mechanism likely contributes to the efficient compaction of the entire sperm genome.