Sir2 and Fun30 Regulate Ribosomal DNA Replication Timing by Modulating Mcm Helicase Positioning and Nucleosome Occupancy

Core Concepts

The histone deacetylase Sir2 and the chromatin remodeling enzyme Fun30 regulate replication timing at the ribosomal DNA (rDNA) by controlling the positioning and activation of the Mcm replicative helicase complex.

Abstract

The study investigates the mechanism by which Sir2 and Fun30 regulate replication timing at the ribosomal DNA (rDNA) in Saccharomyces cerevisiae. Key insights: Deletion of SIR2 leads to derepression of C-pro transcription, which pushes the Mcm helicase complex from its normal location abutting a high-occupancy nucleosome to a nearby region with lower nucleosome occupancy. The displaced Mcm complexes in sir2 mutants are more prone to activation and firing compared to the non-displaced Mcm complexes. The chromatin remodeling activity of Fun30 is required to maintain the low nucleosome occupancy around the displaced Mcm complexes in sir2 mutants. Deletion of FUN30 increases nucleosome occupancy in this region and suppresses the early firing of the displaced Mcm complexes. The authors propose that the displaced Mcm complexes in sir2 mutants represent an exaggerated version of the normal replication initiation mechanism at the rDNA, where a small fraction of Mcm complexes are displaced by low-level C-pro transcription and preferentially activated. This study provides the first in vivo demonstration of a specific chromatin remodeler, Fun30, modulating the activity of a replication origin by regulating the local nucleosome environment around the loaded Mcm helicase complex.

Stats

The rDNA in wild type cells consists of approximately 150 copies. Deletion of FUN30 in a sir2 mutant reduces the rDNA array size to around 40 copies.

Quotes

"Deletion of FUN30 in a sir2 mutant leads to increased nucleosome occupancy at the +2 and +3 positions adjacent to the displaced Mcm complexes, and this increased nucleosome occupancy suppresses the early firing of the displaced Mcm complexes." "Our findings contribute to a growing body of evidence highlighting the importance of positioning of the Mcm replicative helicase within the local nucleosome environment in determining replication timing."

Key Insights Distilled From

Sir2 and Fun30 regulate ribosomal DNA replication timing via Mcm helicase positioning and nucleosome occupancy

by Lichauco,C.,... at www.biorxiv.org 03-26-2024

https://www.biorxiv.org/content/10.1101/2024.03.21.586113v1

Deeper Inquiries

How might the insights from this study on rDNA replication regulation apply to replication dynamics at other genomic loci?

The insights gained from this study on rDNA replication regulation can have broader implications for understanding replication dynamics at other genomic loci. The findings suggest that the positioning of the Mcm replicative helicase within the local nucleosome environment plays a crucial role in determining replication timing. This implies that the local chromatin landscape, particularly nucleosome occupancy and positioning, can influence the activation of replication origins. At other genomic loci, similar mechanisms may be at play where the positioning of the Mcm complex relative to nucleosomes could impact the timing of replication initiation. Different chromatin environments may create varying levels of accessibility for the Mcm complex, affecting its firing propensity. Therefore, the regulation of replication dynamics at other loci may also involve the interplay between chromatin structure, chromatin remodelers, and the activation of replication origins. Understanding how chromatin structure influences replication dynamics at specific genomic loci can provide insights into the regulation of DNA replication throughout the genome. It could help uncover the mechanisms that govern the timing and coordination of replication initiation events, contributing to a more comprehensive understanding of genome replication dynamics.

What other chromatin remodeling enzymes or histone modifications could potentially influence Mcm helicase positioning and activation at replication origins?

Several other chromatin remodeling enzymes and histone modifications could potentially influence Mcm helicase positioning and activation at replication origins. Some of the key players in chromatin remodeling and histone modifications that could impact Mcm helicase dynamics include: SWI/SNF Complex: The SWI/SNF complex is a well-known chromatin remodeler that can alter nucleosome positioning and facilitate access to DNA-binding proteins. It could potentially influence Mcm helicase positioning by modulating nucleosome positioning at replication origins. ISWI Complex: The ISWI (Imitation SWItch) complex is another chromatin remodeler that plays a role in nucleosome sliding and spacing. It could affect Mcm helicase activation by regulating nucleosome positioning near replication origins. Histone Acetylation: Histone acetylation, catalyzed by histone acetyltransferases (HATs), is associated with open chromatin and increased transcriptional activity. Acetylation of histones near replication origins could impact Mcm helicase activation by modulating nucleosome structure and accessibility. Histone Methylation: Histone methylation, catalyzed by histone methyltransferases, can lead to both activation and repression of gene expression. Specific histone methylation marks near replication origins could influence Mcm helicase positioning and activation. Histone Phosphorylation: Histone phosphorylation is involved in various cellular processes, including DNA replication. Phosphorylation of histones near replication origins could potentially affect Mcm helicase dynamics and replication initiation. These chromatin remodeling enzymes and histone modifications, among others, contribute to the dynamic regulation of chromatin structure and gene expression. By influencing nucleosome positioning and chromatin accessibility, they could play a role in modulating Mcm helicase positioning and activation at replication origins.

Given the links between rDNA replication, genome stability, and aging, how could modulating the activities of Sir2 and Fun30 be leveraged for therapeutic interventions in age-related diseases?

The links between rDNA replication, genome stability, and aging highlight the potential significance of modulating the activities of Sir2 and Fun30 for therapeutic interventions in age-related diseases. Here are some ways in which targeting Sir2 and Fun30 could be leveraged for therapeutic purposes: Enhancing Genome Stability: By regulating rDNA replication timing and maintaining proper nucleosome occupancy, Sir2 and Fun30 could help preserve genome stability. Modulating their activities to optimize replication dynamics and chromatin structure may reduce the accumulation of DNA damage and mutations associated with aging. Anti-Aging Strategies: Targeting Sir2, a key regulator of aging processes, could have anti-aging effects by promoting healthy DNA replication and chromatin organization. Activating Sir2 or enhancing its function may counteract age-related changes in genome stability and cellular function. Cancer Therapies: Dysregulation of DNA replication and chromatin structure is a hallmark of cancer. Modulating the activities of Sir2 and Fun30 to maintain proper replication dynamics and chromatin organization could be explored as a strategy for cancer therapy. Targeting these factors could help restore normal replication patterns and inhibit cancer cell proliferation. Epigenetic Therapies: Sir2 and Fun30 are involved in epigenetic regulation, influencing chromatin structure and gene expression. Modulating their activities could be utilized in epigenetic therapies to correct aberrant chromatin states associated with age-related diseases and cancer. Drug Development: Understanding the molecular mechanisms by which Sir2 and Fun30 regulate rDNA replication and chromatin structure could lead to the development of targeted drugs that modulate these pathways. Small molecules or compounds that mimic or inhibit the activities of Sir2 and Fun30 could be explored for therapeutic interventions in age-related diseases. Overall, targeting Sir2 and Fun30 to regulate rDNA replication and chromatin dynamics holds promise for therapeutic interventions aimed at promoting genome stability, combating aging-related changes, and potentially addressing diseases associated with aberrant DNA replication and chromatin structure.