Orc5 induces large-scale chromatin decondensation in a GCN5-dependent manner

Abstract

In eukaryotes, origin recognition complex (ORC) proteins establish the pre-replicative complex ( preRC) at the origins, and this is essential for the initiation of DNA replication. Open chromatin structures regulate the efficiency of preRC formation and replication initiation. However, the molecular mechanisms that control chromatin structure, and how the preRC components establish themselves on the chromatin remain to be understood. In human cells, the ORC is a highly dynamic complex with many separate functions attributed to sub-complexes or individual subunits of the ORC, including heterochromatin organization, telomere and centromere function, centrosome duplication and cytokinesis. We demonstrate that human Orc5, unlike other ORC subunits, when ectopically tethered to a chromatin locus, induces large-scale chromatin decondensation, predominantly during G1 phase of the cell cycle. Orc5 associates with the H3 histone acetyl transferase GCN5 (also known as KAT2A), and this association enhances the chromatin-opening function of Orc5. In the absence of Orc5, histone H3 acetylation is decreased at the origins. We propose that the ability of Orc5 to induce chromatin unfolding during G1 allows the establishment of the preRC at the origins.

The ORC ubiquitin ligase OBI1 promotes DNA replication origin firing

“…Mechanistically, multi-mono-ubiquitylation of the ORC complex may also alter the local chromatin environment around origins, making them more accessible to limiting firing factors. Consistent with this idea, it was recently reported that ORC5 interacts with the histone acetyl-transferase GCN5/KAT2A, making origins more accessible for activation 36 . ORC5 multi-mono-ubiquitylation could promote this interaction, opening the local origin chromatin environment and stimulating origin activation.…”

A human cancer cell line initiates DNA replication normally in the absence of ORC5 and ORC2 proteins

“…Subunits of ORC have also been implicated in centrosome biology or cytokinesis (31)(32)(33) so it would be interesting to examine whether these functions account for slow proliferation. In addition, we are currently investigating whether other functions of ORC, like its role in epigenetics (34)(35)(36), is responsible for the decrease in proliferation despite the near normal replication initiation. As we have noted, decrease of ORC subunits decreases cell proliferation without affecting replication initiation.…”

Metazoan DNA replication origins

“…In higher eukaryotes, the positioning of replication origins is not dictated by a single DNA sequence motif and appears to be more dependent on the chromatin environment. For instance, it has been suggested that ORC triggers a permissive chromatin environment through the direct recruitment of histone acetyltransferases, such as GCN5 [33]; [34]). Moreover, the OGRE/G4 element upstream of the pre-RC is nucleosome-free, whereas the IS contains a labile nucleosome that might be removed during the initiation step.…”