Polycomb response elements mediate the formation of chromosome higher-order structures in the bithorax complex

Area of Science:

• Developmental Biology
• Epigenetics
• Genomics

Background:

• Polycomb group genes (PcGs) and Polycomb response elements (PREs) are essential for maintaining gene silencing across cell divisions.
• PREs are hypothesized to mediate transcriptional memory by forming higher-order nuclear structures.

Purpose of the Study:

• To investigate the in vivo three-dimensional structure of the Drosophila bithorax complex (BX-C) in relation to epigenetic silencing.
• To determine the role of higher-order chromatin structures in maintaining transcriptional states.

Main Methods:

• Chromosome conformation capture (3C) was employed to analyze DNA interactions.
• Fluorescent in situ hybridization (FISH) and FISH-immunostaining (FISH-I) were used to visualize nuclear organization and protein binding.

Main Results:

• In the repressed state, key elements including PREs and core promoters within the BX-C interact, forming a complex three-dimensional structure.
• This higher-order structure is critical for the epigenetic silencing of the BX-C.
• Stable alternative transcription states require substantial alterations in these higher-order structures.

Conclusions:

• The study demonstrates that complex, long-range interactions form higher-order structures essential for Polycomb-mediated epigenetic silencing of the BX-C.
• While histone modifications and reduced PcG levels can initiate an epigenetic switch, stable heritability of alternative transcription states depends on significant changes in nuclear architecture.