To start copying a particular region, the cell machinery that replicates DNA binds to a sequence known as the origin of replication. Cells follow precisely controlled programs to replicate distinct regions of chromosomes at different times. Within human, fly and other eukaryotic nuclei, DNA is packaged into structures known as chromosomes.
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Cells carefully maintain and repair their DNA, and typically make a complete copy of the genome before they divide to ensure that after division, each daughter cell has a full set. Inside cells, molecules of DNA provide the instructions needed to make proteins. They suggest a mechanism for the establishment of late replication that does not depend on an asynchronous firing of late replication origins. Our findings implicate a SNF2 family ATP-dependent motor protein SUUR in the insulator function, reveal that DNA replication can be delayed by a chromatin barrier, and uncover a critical role for architectural proteins in replication control. Thus, SUMM4 can impart late replication of intercalary heterochromatin by attenuating the progression of replication forks through euchromatin/heterochromatin boundaries. Furthermore, SuUR or mod(mdg4) mutations reverse underreplication of intercalary heterochromatin. SUUR and Mod(Mdg4)-67.2 together mediate the activities of gypsy insulator that prevent certain enhancer–promoter interactions and establish euchromatin–heterochromatin barriers in the genome. Mod(Mdg4) stimulates SUUR ATPase activity and is required for a normal spatiotemporal distribution of SUUR in vivo. Here, we developed a novel method termed MS-Enabled Rapid protein Complex Identification (MERCI) to isolate a stable stoichiometric native complex SUMM4 that comprises SUUR and a chromatin boundary protein Mod(Mdg4)-67.2.
SUUR negatively regulates DNA replication fork progression however, its mechanism of action remains obscure. Drosophila SNF2-related factor SUUR imparts locus-specific underreplication of polytene chromosomes. Intercalary heterochromatin domains replicate very late in both diploid chromosomes of dividing cells and in endoreplicating polytene chromosomes where they are also underreplicated. Asynchronous replication of chromosome domains during S phase is essential for eukaryotic genome function, but the mechanisms establishing which domains replicate early versus late in different cell types remain incompletely understood.
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