Association of the Origin Recognition Complex with Heterochromatin and HP1 in Higher Eukaryotes

Abstract

The origin recognition complex (ORC) is required to initiate eukaryotic DNA replication and also engages in transcriptional silencing in S. cerevisiae. We observed a striking preferential but not exclusive association of Drosophila ORC2 with heterochromatin on interphase and mitotic chromosomes. HP1, a heterochromatin-localized protein required for position effect variegation (PEV), colocalized with DmORC2 at these sites. Consistent with this localization, intact DmORC and HP1 were found in physical complex. The association was shown biochemically to require the chromodomain and shadow domains of HP1. The amino terminus of DmORC1 contained a strong HP1-binding site, mirroring an interaction found independently in Xenopus by a yeast two-hybrid screen. Finally, heterozygous DmORC2 recessive lethal mutations resulted in a suppression of PEV. These results indicate that ORC may play a widespread role in packaging chromosomal domains through interactions with heterochromatin-organizing factors.

A human homolog of Drosophila lethal(3)malignant brain tumor (l(3)mbt) protein associates with condensed mitotic chromosomes

“…For example, HP1 (heterochromatin protein 1), which carries a motif called chromo-domain that is also present in Pc protein, is a structural component of heterochromatin and is necessary for chromosome condensation and segregation (Kellum et al, 1995;Pak et al, 1997). CCF (centrosomal and chromosomal factor), which shares homology to the chromatin-binding sites of the Psc protein, is required for the proper condensation of mitotic chromosomes (Kodjabachian et al, 1998).…”

DNA replication licensing and cell cycle kinetics of oligodendroglial tumours

“…We have examined expression of the standard proliferation marker Ki67, the DNA replicationlicensing factor Mcm2 and the repressor of origin-licensing Geminin during the proliferative cell cycle, using a novel, nonchemical method for cell synchronisation (membrane elution; Thornton et al, 2002;Helmstetter et al, 2003; Figure 1C). We have employed membrane elution because use of chemical methods for cell synchronisation may account in part for reported discrepancies in temporal periodicities and subcellular localisation of replication proteins such as Orc1 (Pak et al, 1997;Kreitz et al, 2001;Okuno et al, 2001;Li and DePamphilis, 2002) and Cdc6 (Fujita et al, 1999;Jiang et al, 1999;Mendez and Stillman, 2000;Petersen et al, 2000).…”

Perpetuating the double helix: molecular machines at eukaryotic DNA replication origins

“…(17) Although the replicator sequences in other eukarya are larger and less defined than in S. cerevisiae, ORC has been conserved during evolution and is required for initiation of DNA replication in different species. (16) In Xenopus, Drosophila melanogaster and mammalian cells, ORC associates with the chromatin, (18)(19)(20)(21)(22) but its binding sites are poorly defined. In flies, a subset of ORCbinding sites have been identified at loci that promote developmentally regulated DNA amplification through repeated rounds of replication.…”