Sister chromatid cohesion couples DNA replication in Saccharomyces cerevisiae

Abstract

Sister chromatids are tethered together by the cohesin ring complex from S phase to the onset of anaphase, ensuring faithful chromosome segregation. Defects in sister chromatid cohesion result in premature sister chromatid separation and chromosome loss, which may lead to aneuploidy and chromosome instability (CIN), as commonly seen in solid tumors. The process of sister chromatid cohesion establishment has been proposed to intimately couple with DNA replication. However, the regulatory mechanism behind this model remains unclear. In Saccharomyces cerevisiae, Bre1 is a conserved RING domain-containing E3 ubiquitin ligase that interacts with Lge1, a protein required for cell size control. Deletion mutant of Bre1 or Lge1 shows whole chromosome instability. However, the underlying causes are currently unknown. Bre1 and Lge1 are responsible for histone H2B monoubiquitination (H2Bub1), which regulates transcription, DNA replication, DNA damage response and repair, ensuring the structural integrity of chromosomes. Defects in H2Bub1 have been observed in various tumors. Here we discovered that Bre1, Lge1 and H2Bub1 play a novel role in chromosome segregation and sister chromatid cohesion. Bre1’s function in G1 and S phases contributes to sister chromatid cohesion establishment, but it is not required for cohesion maintenance in G2 phase, or for loading of cohesin components onto chromatin in G1. However, Bre1 regulates the localization of an upstream replication factor Minichromsome maintenance protein 10 (Mcm10) and cohesion establishment factors Ctf4, Ctf18 and Eco1 to early replication origins in G1 and S phases, and promotes cohesin subunit Smc3 acetylation for facilitating cohesion establishment and stabilization. Consistently, Bre1’s catalytic RING domain mutant and H2B mutant defective in ubiquitination show similar defects in recruiting Mcm10 and Ctf4 to the early replication origins in S phase as in bre1 mutant. These findings shed light on how Bre1 and H2Bub1 regulate cohesion during the cell cycle, and suggest that Bre1-induced H2Bub1 epigenetically marks the replication origins to signal the recruitment of replication factors and cohesion establishment factors to origins, thereby coupling DNA replication and cohesion establishment in S phase to ensure chromosome stability.