H2B mono-ubiquitylation facilitates fork stalling and recovery during replication stress by coordinating Rad53 activation and chromatin assembly.

The influence of mono-ubiquitylation of histone H2B (H2Bub) on transcription via nucleosome reassembly has been widely documented. Recently, it has also been shown that H2Bub promotes recovery from replication stress; however, the underling molecular mechanism remains unclear. Here, we show that H2B ubiquitylation coordinates activation of the intra-S replication checkpoint and chromatin re-assembly, in order to limit fork progression and DNA damage in the presence of replication stress. In particular, we show that the absence of H2Bub affects replication dynamics (enhanced fork progression and reduced origin firing), leading to γH2A accumulation and increased hydroxyurea sensitivity. Further genetic analysis indicates a role for H2Bub in transducing Rad53 phosphorylation. Concomitantly, we found that a change in replication dynamics is not due to a change in dNTP level, but is mediated by reduced Rad53 activation and destabilization of the RecQ helicase Sgs1 at the fork. Furthermore, we demonstrate that H2Bub facilitates the dissociation of the histone chaperone Asf1 from Rad53, and nucleosome reassembly behind the fork is compromised in cells lacking H2Bub. Taken together, these results indicate that the regulation of H2B ubiquitylation is a key event in the maintenance of genome stability, through coordination of intra-S checkpoint activation, chromatin assembly and replication fork progression.

Histone ubiquitylation and chromatin dynamics.

Histones are subject to several post-translational modifications, which act to regulate gene expression and other processes on the DNA template. One such modification is the addition of a single ubiquitin moiety, which has been reported to influence chromatin dynamics and exhibit cross-talk with other histone modifications. Mono-ubiquitylation of H2B has been reported in eukaryotes as divergent as budding yeast, flies and humans, and is linked to transcriptional activation and gene silencing. Furthermore, ubiquitylation of H2A is also important for transcriptional repression in higher eukaryotes, and both histones play key roles in DNA repair. In this review, we will give an overview of the enzymes important for ubiquitylation and deubiquitylation of the various histone species, before examining the role of ubiquitylated histones in shaping the chromatin landscape and thus controlling the accessibility of DNA to effector proteins, through putative roles in promoting histone-histone interactions and stabilizing the structure of nucleosomes. We will finally discuss other processes reported to involve ubiquitylation of histones, including DNA repair, recombination and mRNA processing, underlining the diverse actions of these modifications.

Flickin' the ubiquitin switch: the role of H2B ubiquitylation in development.

The reversible ubiquitylation of histone H2B has long been implicated in transcriptional activation and gene silencing. However, many questions regarding its regulation and effects on chromatin structure remain unanswered. In addition, while several studies have uncovered an involvement of this modification in the control of certain developmental processes, a more general understanding of its requirement is lacking. Herein, we present a broad overview of the pathways known to be regulated by H2B ubiquitylation, while drawing parallels between findings in disparate organisms, in order to facilitate continued delineation of its spatiotemporal role in development. Finally, we integrate the findings of recent studies into how H2B ubiquitylation affects chromatin, and cast an eye over emerging areas for future research.