DNA-bridging by an archaeal histone variant via a unique tetramerisation interface

Area of Science:

• Structural biology
• Molecular biology
• Genomics

Background:

• Eukaryotic histones form heterodimers (H3/H4, H2A/H2B) into nucleosomes.
• Archaeal histones are dimeric, forming 'hypernucleosomes' with DNA.
• The function of archaeal histone variants remains largely unknown.

Purpose of the Study:

• To characterize the structure and function of the archaeal histone paralogue MJ1647 from Methanocaldococcus jannaschii.
• To investigate the role of its unique C-terminal extension.
• To understand its implications for archaeal genome structure and gene expression.

Main Methods:

• X-ray crystallography (1.9 Å resolution) to determine dimeric MJ1647 structure.
• Structural modeling to predict tetramer formation.
• Site-directed mutagenesis to analyze functional domains.
• Single-molecule tethered particle motion assays.
• DNA binding assays.

Main Results:

• MJ1647 possesses a unique C-terminal extension enabling homotetramerization.
• The tetramerization module involves two alpha helices in a handshake arrangement.
• MJ1647 tetramers exhibit DNA bridging capability, unlike canonical histones.
• Tetramers bind approximately 60 bp of DNA and cooperatively compact DNA.
• MJ1647 inhibits transcription machinery access to promoters in vitro.

Conclusions:

• MJ1647 is the first identified histone with DNA bridging properties.
• These properties suggest a significant role in archaeal genome organization.
• MJ1647's function has implications for regulating gene expression in archaea.