SMC complexes can traverse physical roadblocks bigger than their ring size

Area of Science:

• Molecular Biology
• Chromatin Dynamics
• Biophysics

Background:

• Ring-shaped structural maintenance of chromosomes (SMC) complexes, including condensin and cohesin, are known to extrude DNA loops.
• The mechanism by which these complexes navigate and extrude DNA in the presence of protein-bound chromatin remains poorly understood.

Purpose of the Study:

• To investigate the ability of yeast condensin and cohesin to extrude DNA loops in the presence of various roadblocks.
• To elucidate the translocation mechanism of SMC complexes during DNA loop extrusion.

Main Methods:

• In vitro single-molecule visualization techniques were employed.
• Experiments utilized yeast condensin and cohesin, along with nucleosomes, RNA polymerase, dCas9, and DNA-bound nanoparticles as obstacles.

Main Results:

• Nucleosomes, RNA polymerase, and dCas9 did not impede yeast condensin-mediated loop extrusion.
• Large DNA-bound nanoparticles (up to 200 nm) were translocated into DNA loops by both condensin and cohesin.
• Even a non-opening single-chain cohesin variant could accommodate large roadblocks during extrusion.
• Extruded DNA does not pass topologically through the SMC ring.

Conclusions:

• SMC-driven DNA loop extrusion can readily accommodate large obstacles, challenging previous assumptions.
• The findings support a non-topological mechanism for DNA loop extrusion by SMC complexes.