The promoter-poised Rpd3 HDAC complex orchestrates global chromatin reprogramming upon nutrient transition

This study reveals that the Rpd3L histone deacetylase complex acts as a metabolic gatekeeper in *Saccharomyces cerevisiae* by dynamically removing H3K9 acetylation from active promoters upon nutrient shifts, thereby coordinating the shutdown of growth programs and ensuring transcriptional fidelity during metabolic transitions.

The Gist

The Big Picture: The Cell's "Metabolic Switch"

Imagine a yeast cell as a busy restaurant kitchen.

• Glucose (Sugar) is like a massive delivery of fresh, easy-to-cook ingredients. When the kitchen has plenty of this, the chefs (genes) work fast, making lots of bread and pasta (growth and fermentation).
• Starvation (No Sugar) is like the delivery truck breaking down. The kitchen must switch gears immediately. The chefs need to stop making bread and start cooking a slow, complex stew using whatever leftovers are in the pantry (respiration and fat burning).

The problem? Kitchens are messy. If the chefs don't switch off the "bread-making" machines quickly enough, or if they don't turn on the "stew-making" machines fast enough, the restaurant fails.

This paper discovers the manager who ensures this switch happens perfectly. That manager is a protein called Rpd3.

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The Main Character: Rpd3 (The "Eraser")

In the world of genetics, genes have "on" and "off" switches.

• Acetylation is like putting a bright green "ON" sticker on a gene. It tells the cell, "Make this protein!"
• Deacetylation is like taking that sticker off. It tells the cell, "Stop making this."

For a long time, scientists were confused about Rpd3. They knew it was an "Eraser" (it removes the green stickers), but they kept finding it hanging out at genes that were currently ON and working hard. It seemed like a contradiction: Why would an eraser stand next to a sign that says "DO NOT ERASE"?

The Paper's Discovery:
Rpd3 isn't just an eraser; it's a poised security guard. It stands right next to the "ON" switches of growth genes, waiting. As long as there is plenty of sugar, it stays quiet. But the moment the sugar runs out, it instantly swipes away the green stickers, shutting down the growth programs so the cell can switch to survival mode.

The Two Teams: Rpd3L and Rpd3S

Rpd3 doesn't work alone. It has two different "uniforms" or teams, each with a specific job:

1. The "Gatekeeper" Team (Rpd3L):

   • Job: They stand at the front door (the promoter) of the genes.
   • Action: When the cell needs to stop growing, this team rushes to the front doors of the growth genes and locks them tight. They are the ones who ensure the cell doesn't keep trying to grow when it's starving.
   • Key Member: A subunit named Pho23 acts like the keyholder. Without Pho23, the doors stay unlocked, and the cell keeps trying to grow even when it's starving.

2. The "Cleanup Crew" Team (Rpd3S):

   • Job: They patrol the inside of the building (the gene body).
   • Action: Their job is to make sure the inside of the gene is clean and quiet. They prevent "noise" or random, messy signals from starting inside the gene (called cryptic transcription).
   • Key Member: A subunit named Eaf3 leads this crew. If you remove them, the gene gets a little messy, but the main "ON/OFF" switch at the door still works.

The Dance with Gcn5 (The "Sticker Paster")

There is another protein called Gcn5. If Rpd3 is the eraser, Gcn5 is the sticker paster.

• Gcn5 puts the green "ON" stickers on genes to turn them on.
• The paper shows that Rpd3 and Gcn5 are often standing next to each other at the same genes.

How the switch works:

• When Sugar is High: Gcn5 is busy putting stickers on growth genes. Rpd3 is standing by, waiting.
• When Sugar Runs Out: Gcn5 steps back and leaves. Rpd3 immediately steps in and erases the stickers.
• The Result: The growth genes turn off instantly.

If Rpd3 is missing, Gcn5 leaves, but the stickers stay on the growth genes. The cell gets confused, keeps trying to grow, and fails to switch to survival mode.

Why This Matters: Solving a Mystery

For years, scientists were puzzled by the "Paradox of the Active Promoter": Why do repressors (erasers) hang out at active genes?

This paper solves the mystery. Rpd3 isn't there to stop the gene from working right now. It's there to be ready. It's like a fire extinguisher hanging next to a busy stove. You don't need it while you are cooking, but the moment a fire starts (a nutrient change), you need it right there to put it out immediately.

The Bottom Line

This research shows that cells have a sophisticated, two-part system to manage their energy:

1. Rpd3L (The Gatekeeper) waits at the door to instantly shut down growth when food runs out.
2. Rpd3S (The Cleanup Crew) keeps the inside of the genes tidy.

Together, they act as a metabolic gatekeeper, ensuring that the cell's genetic instructions match its current reality. Without them, the cell would be like a driver trying to race a car while the brakes are stuck on—it would be chaotic, inefficient, and eventually, the engine would burn out.

Technical Summary

1. Problem Statement

• Metabolic Adaptation: Saccharomyces cerevisiae must rapidly reprogram its transcriptional landscape to switch between fermentative growth (glucose-rich) and respiratory growth (glucose-depleted). This involves shutting down growth programs (e.g., ribosome biogenesis) and activating catabolic pathways (e.g., fatty acid oxidation).
• The HDAC Paradox: While Histone Deacetylases (HDACs) like Rpd3 are classically viewed as transcriptional repressors, genome-wide studies have consistently shown they are enriched at actively transcribed genes. This contradicts the classical model where HDACs are only found at silenced loci.
• Knowledge Gap: The mechanisms by which HDACs coordinate with Histone Acetyltransferases (HATs) like Gcn5 to dynamically remodel chromatin during nutrient shifts, and the specific biological significance of HDACs at active promoters, remain unclear. Specifically, how Rpd3 balances global deacetylation with local gene regulation is unknown.

2. Methodology

The authors employed a multi-omics approach using S. cerevisiae to investigate chromatin dynamics during glucose depletion:

• Strain Construction: Generation of deletion strains for RPD3, Gcn5, and specific subunits of Rpd3 complexes: Pho23 (Rpd3L-specific), Eaf3 (Rpd3S-specific), and Rco1.
• Nutrient Shift Experiments: Cells were grown in glucose-replete media (+D) and shifted to glucose-depleted media (-D) for 30 minutes to capture early transcriptional and chromatin responses.
• Genomic Assays:
  • RNA-seq: To profile transcriptome changes in WT and mutant strains under +D and -D conditions.
  • ChIP-seq: Performed for Rpd3, Gcn5, Pho23, Eaf3, and histone marks (H3K9ac, H3ac) to map genome-wide occupancy and histone modification landscapes.
  • Western Blotting: To assess global levels of histone acetylation.
  • qPCR: For validation of specific gene expression changes.
• Bioinformatics: Used Galaxy, MACS2 (peak calling), and custom scripts for metagene analysis, differential expression (DESeq2), and integration of ChIP-seq with RNA-seq data.

3. Key Contributions & Findings

A. Rpd3 is the Primary Mediator of Nutrient-Dependent Deacetylation

• Upon glucose depletion, cells undergo rapid, global histone deacetylation.
• Deletion of RPD3 (but not other HDACs) significantly blunted this global loss of H3K9ac and H3ac, identifying Rpd3 as the principal effector of starvation-induced deacetylation.

B. Resolution of the "HDAC Paradox": Promoter-Poised Repression

• Dynamic Binding: Rpd3 binds to both promoters and gene bodies in glucose-rich conditions but shifts almost exclusively to promoters upon glucose depletion.
• Co-occupancy with Gcn5: Rpd3 and the HAT Gcn5 co-occupy promoters of active genes. However, their dynamics are reciprocal:
  • Growth Genes: In glucose, Gcn5 is high and Rpd3 is present. Upon starvation, Gcn5 disengages, and Rpd3 binding increases, leading to H3K9 deacetylation and repression.
  • FAO Genes: In glucose, both are low. Upon starvation, Gcn5 binds to activate, while Rpd3 remains poised or binds to ensure rapid shutdown when glucose returns.
• The "Poised" Model: Rpd3L (the large complex) is pre-bound ("poised") at the promoters of active genes. This allows for immediate transcriptional shutdown upon nutrient stress without waiting for new recruitment, acting as an "epigenetic toggle."

C. Distinct Roles of Rpd3 Complexes (Rpd3L vs. Rpd3S)

The study delineates a clear division of labor between the two major Rpd3 complexes:

1. Rpd3L (Large Complex, subunit Pho23):
   • Function: Mediates promoter-specific repression.
   • Mechanism: Pho23 recruits Rpd3L to promoters of nutrient-responsive genes (e.g., ribosomal genes, INO1, CAT2).
   • Phenotype: Loss of Pho23 mimics the rpd3Δ phenotype: failure to repress growth genes during starvation and failure to induce respiratory genes. This leads to metabolic misexpression and impaired growth on non-fermentable carbon sources (ethanol).
2. Rpd3S (Small Complex, subunit Eaf3):
   • Function: Mediates global/gene-body deacetylation.
   • Mechanism: Recognizes H3K36me3 on gene bodies to suppress cryptic transcription and maintain transcriptional fidelity.
   • Phenotype: Loss of Eaf3 causes a modest retention of global H3K9ac but does not result in the specific promoter derepression seen in rpd3Δ or pho23Δ.

D. Metabolic Implications

• Transcriptional Fidelity: Rpd3L ensures that growth programs (ribosome biogenesis) are rapidly silenced during starvation, while Rpd3S prevents aberrant transcription within coding regions.
• Metabolic Recycling: The authors propose that global deacetylation by Rpd3 may liberate acetate, which is converted to acetyl-CoA by Acs2, potentially fueling the reacetylation of promoters required for the starvation response (a metabolic feedback loop).

4. Results Summary

• Transcriptomics: rpd3Δ and pho23Δ strains fail to repress ribosomal/ribosome biogenesis genes upon glucose starvation and fail to fully induce respiratory genes. Conversely, they show aberrant expression of fatty acid oxidation (FAO) genes even in glucose-rich conditions.
• Chromatin Dynamics: In WT cells, H3K9ac is removed from ribosomal promoters upon starvation. In rpd3Δ and pho23Δ, this deacetylation is blocked, and H3K9ac persists, correlating with continued transcription.
• Complex Specificity: Pho23 deletion specifically reduces Rpd3 occupancy at promoters of nutrient-regulated genes, whereas Eaf3 deletion affects gene body occupancy but not promoter repression.

5. Significance

• Redefining HDAC Function: The paper resolves the paradox of HDAC enrichment at active promoters by proposing a "promoter-poised" model. HDACs are not just static silencers but dynamic regulators that maintain genes in a state ready for rapid repression.
• Metabolism-Chromatin Coupling: It provides a mechanistic link between nutrient sensing (glucose availability) and chromatin remodeling, demonstrating how metabolic cues directly dictate the balance between HAT and HDAC activities to rewire the transcriptome.
• Complex Specialization: It clarifies that while Rpd3 is a single enzyme, its function is bifurcated by its complex partners (Pho23 vs. Eaf3) to handle distinct tasks: local promoter switching (Rpd3L) vs. global chromatin homeostasis (Rpd3S).
• Broader Relevance: This mechanism likely extends to higher eukaryotes, where HDACs similarly integrate metabolic and signaling pathways to ensure transcriptional fidelity during environmental stress.

In conclusion, the study establishes Rpd3L as a critical "metabolic gatekeeper" that couples nutrient status to chromatin reprogramming, ensuring that yeast cells can rapidly and reversibly switch between growth and survival modes.