Insulator protein Su(Hw) recruits SAGA and Brahma complexes and constitutes part of Origin Recognition Complex-binding sites in the Drosophila genome

Abstract

Despite increasing data on the properties of replication origins, molecular mechanisms underlying origin recognition complex (ORC) positioning in the genome are still poorly understood. The Su(Hw) protein accounts for the activity of best-studied Drosophila insulators. Here, we show that Su(Hw) recruits the histone acetyltransferase complex SAGA and chromatin remodeler Brahma to Su(Hw)-dependent insulators, which gives rise to regions with low nucleosome density and creates conditions for ORC binding. Depletion in Su(Hw) leads to a dramatic drop in the levels of SAGA, Brahma and ORC subunits and a significant increase in nucleosome density on Su(Hw)-dependent insulators, whereas artificial Su(Hw) recruitment itself is sufficient for subsequent SAGA, Brahma and ORC binding. In contrast to the majority of replication origins that associate with promoters of active genes, Su(Hw)-binding sites constitute a small proportion (6%) of ORC-binding sites that are localized preferentially in transcriptionally inactive chromatin regions termed BLACK and BLUE chromatin. We suggest that the key determinants of ORC positioning in the genome are DNA-binding proteins that constitute different DNA regulatory elements, including insulators, promoters and enhancers. Su(Hw) is the first example of such a protein.

Interleukin-2 improves amyloid pathology, synaptic failure and memory in Alzheimer’s disease mice

“…Moreover, Gcn5 co-occurs with CP190 at CP190 standalone sites (Figure 3). Previously, the occurrence of SAGA components ENY2, Ada2b or of Gcn5 has been detected at selected dCTCF or Su(Hw) sites (20,52,53), which now may be explained in part due to CP190 binding at these sites in addition to direct contacts with dCTCF or Su(Hw). Nucleic Acids Research, 2017, Vol.…”

The chromatin environment shapes DNA replication origin organization and defines origin classes

“…In support of this hypothesis, G-rich elements were identified as a binding site for ORC in mouse rDNA (Zellner et al 2007), and a recent biochemical analysis showed that purified ORC could bind to G4 RNA or DNA (Hoshina et al 2013). In the Drosophila genome, ORC binding sites contain a central GC-rich region surrounded by AT-rich sequences (Vorobyeva et al 2013). G-rich motifs/G4 may help in destabilizing the proximal double-stranded DNA structure (König et al 2013), facilitating the MCM2-7 helicase access to the leading single-stranded DNA template.…”

Nonreplicative functions of the origin recognition complex

“…It is likely, that in the heterochromatin, ORC also binds to the islands of ‘locally open’ chromatin. For example, in the heterochromatin of Drosophila , ORC binds predominantly to the insulators of the Su(Hw) type [95]. …”