An extrinsic motor directs chromatin loop formation by cohesin

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• The cohesin complex is crucial for sister chromatid cohesion and interphase chromatin organization.
• Cohesin's role in chromatin loop formation is often attributed to the 'loop extrusion' hypothesis.
• The in vivo relevance of loop extrusion for chromatin looping remains experimentally unverified.

Purpose of the Study:

• To investigate the mechanism of in vivo chromatin loop formation by cohesin.
• To test whether cohesin-mediated loop extrusion is essential for chromatin loop formation.
• To elucidate the relationship between cohesin function, transcription, and genome organization.

Main Methods:

• Generation of cohesin variants in budding yeast (S. cerevisiae) with impaired DNA loop extrusion but intact DNA topological entrapment.
• Analysis of chromatin loop formation in these variants.
• Investigating the role of transcription in cohesin-mediated loop formation.

Main Results:

• In vivo chromatin loops form independently of cohesin's DNA loop extrusion activity.
• Transcription actively promotes and expands chromatin loops, influencing their positions.
• Cohesin variants lacking loop extrusion capability still form loops, indicating an alternative mechanism.

Conclusions:

• The 'loop extrusion' hypothesis may not fully explain in vivo chromatin loop formation.
• Cohesin likely forms chromatin loops through DNA-DNA capture at transcription sites.
• This DNA capture mechanism unifies cohesin's roles in sister chromatid cohesion and interphase genome organization.