Structure and function of the Orc1 BAH-nucleosome complex

Pablo De Ioannes,Jef D Boeke,Zheng Kuang,Victor A Leon,Miao Wang,Andreas Hochwagen,Karim-Jean Armache

doi:10.1038/s41467-019-10609-y

Abstract

The Origin Recognition Complex (ORC) is essential for replication, heterochromatin formation, telomere maintenance and genome stability in eukaryotes. Here we present the structure of the yeast Orc1 BAH domain bound to the nucleosome core particle. Our data reveal that Orc1, unlike its close homolog Sir3 involved in gene silencing, does not appear to discriminate between acetylated and non-acetylated lysine 16, modification states of the histone H4 tail that specify open and closed chromatin respectively. We elucidate the mechanism for this unique feature of Orc1 and hypothesize that its ability to interact with nucleosomes regardless of K16 modification state enables it to perform critical functions in both hetero- and euchromatin. We also show that direct interactions with nucleosomes are essential for Orc1 to maintain the integrity of rDNA borders during meiosis, a process distinct and independent from its known roles in silencing and replication.

Highlights

The Origin Recognition Complex (ORC) is essential for replication, heterochromatin formation, telomere maintenance and genome stability in eukaryotes
We discovered that direct interactions of Orc[1] with nucleosomes are essential to maintaining the integrity of ribosomal DNA (rDNA) borders during meiosis
As this residue is conserved between Orc[1] and Sir[3] (Fig. 1a, Top), we expressed the Orc[1] bromo-adjacent homology (BAH) L79I mutant in insect cells for our studies

Summary

Introduction

The Origin Recognition Complex (ORC) is essential for replication, heterochromatin formation, telomere maintenance and genome stability in eukaryotes. While it is clear that ORC binds within chromatin, if and at which loci direct nucleosome interactions are required for its function is not well established One example of such ambiguity corresponds to the protection of rDNA from genomic rearrangements. Orc[1] is involved in protecting these regions, and deletion of its BAH domain leads to increased rates of DSBs and NAHR at this locus, resulting in a perturbed number of rDNA repeats in progeny[8] It is not known whether this function depends on direct interactions between the BAH domain and nucleosomes or on recruitment of another set of proteins via this domain

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Conclusion