Yeast HMO1: Linker Histone Reinvented

Area of Science:

• Molecular Biology
• Epigenetics
• Chromatin Structure and Dynamics

Background:

• Eukaryotic genomes are organized into chromatin, with higher-order structure regulated by linker DNA interactions with histone H1 or high mobility group box (HMGB) proteins.
• Histone H1 typically stabilizes nucleosomes, while HMGB proteins may destabilize chromatin through DNA bending.
• Metazoan histone H1's chromatin compaction relies on its C-terminal domain, unlike the yeast Saccharomyces cerevisiae linker histone Hho1p.

Purpose of the Study:

• To investigate the functions of Saccharomyces cerevisiae HMO1, an HMGB protein with a unique lysine-rich terminal domain.
• To elucidate HMO1's role in stabilizing genomic DNA, nucleosome-free regions, and fragile nucleosomes.
• To compare HMO1's chromatin-stabilizing properties with those of mammalian linker histone H1.

Main Methods:

• Analysis of HMO1's function on ribosomal DNA (rDNA) and ribosomal protein genes.
• Observation of HMO1's role during DNA replication, focusing on DNA junctions and the DNA damage response.
• Assessment of chromatin dynamics and nuclease sensitivity in the presence and absence of HMO1.
• Evaluation of chromatin remodeling during DNA double-strand break repair.

Main Results:

• HMO1 stabilizes nucleosome-free regions and fragile nucleosomes, particularly on rDNA and ribosomal protein genes.
• HMO1 ensures low nucleosome density at DNA junctions during replication, aiding DNA damage response and topoisomerase function.
• HMO1 stabilizes chromatin, similar to mammalian histone H1, evidenced by reduced nuclease sensitivity and faster DNA repair-associated chromatin remodeling.
• HMO1's chromatin stabilization function requires its lysine-rich extension.

Conclusions:

• Saccharomyces cerevisiae HMO1 exhibits unique linker histone-like functions, stabilizing both conventional nucleosome arrays and DNA regions with low nucleosome density.
• HMO1's lysine-rich domain is critical for its chromatin-stabilizing ability, impacting genomic stability and dynamic processes like replication and repair.
• HMO1 represents an evolutionary convergence, possessing linker histone-like properties within the HMGB protein family.