Histone H3 lysine 4 hypermethylation prevents aberrant nucleosome remodeling at the PHO5 promoter

Area of Science:

• Molecular Biology
• Epigenetics
• Yeast Genetics

Background:

• Histone methylation, specifically H3K4me, is implicated in gene transcription regulation.
• The precise mechanism by which H3K4me influences transcription, particularly repression, in Saccharomyces cerevisiae is not fully understood.
• Understanding epigenetic regulation is crucial for deciphering gene expression control.

Purpose of the Study:

• To elucidate the mechanism by which histone H3K4 methylation (H3K4me) mediates transcriptional repression of the PHO5 gene in yeast.
• To investigate the role of H3K4me in regulating chromatin structure and transcription factor binding at the PHO5 promoter.
• To identify the proteins involved in mediating the H3K4me-dependent repression.

Main Methods:

• Genetic manipulation (SET1 deletion) to study PHO5 promoter derepression.
• Chromatin structure analysis to assess nucleosome disassembly and RNA polymerase II (Pol II) binding.
• Co-immunoprecipitation assays to investigate protein interactions.
• Analysis of histone deacetylation activity.

Main Results:

• H3K4me inhibits basal PHO5 transcription by suppressing nucleosome disassembly at the promoter under high-phosphate conditions.
• SET1 deletion leads to a labile chromatin structure, increasing Pol II binding but not transactivator binding (Pho2, Pho4).
• Pho23 and Cti6 proteins recruit the Rpd3L complex to H3K4-methylated promoters, and Rpd3 deacetylation activity is essential for Set1 function.

Conclusions:

• Set1-mediated H3K4 methylation suppresses nucleosome remodeling at the PHO5 promoter, reducing basal transcription under repressive conditions.
• The recruitment of the Rpd3L complex by Pho23 and Cti6 is a key step in H3K4me-dependent repression.
• Restricting aberrant nucleosome remodeling is a critical mechanism for precise control of gene transcription by transactivators.