Conservation of domain structure in a fast-evolving heterochromatic SUUR protein in drosophilids

Regulatory functions and chromatin loading dynamics of linker histone H1 during endoreplication in Drosophila

Eukaryotic DNA replicates asynchronously, with discrete genomic loci replicating during different stages of S phase. Drosophila larval tissues undergo endoreplication without cell division, and the latest replicating regions occasionally fail to complete endoreplication, resulting in underreplicated domains of polytene chromosomes. Here we show that linker histone H1 is required for the underreplication (UR) phenomenon in Drosophila salivary glands. H1 directly interacts with the Suppressor of UR (SUUR) protein and is required for SUUR binding to chromatin in vivo. These observations implicate H1 as a critical factor in the formation of underreplicated regions and an upstream effector of SUUR. We also demonstrate that the localization of H1 in chromatin changes profoundly during the endocycle. At the onset of endocycle S (endo-S) phase, H1 is heavily and specifically loaded into late replicating genomic regions and is then redistributed during the course of endoreplication. Our data suggest that cell cycle-dependent chromosome occupancy of H1 is governed by several independent processes. In addition to the ubiquitous replication-related disassembly and reassembly of chromatin, H1 is deposited into chromatin through a novel pathway that is replication-independent, rapid, and locus-specific. This cell cycle-directed dynamic localization of H1 in chromatin may play an important role in the regulation of DNA replication timing.

The effects of SUUR protein suggest its role in repressive chromatin renewal during replication in Drosophila

Replication of chromosomes is central to heredity. To become available for replication machinery, DNA invariably needs to dissociate from chromatin proteins. Yet, chromatin landscape must be promptly re-established during or soon after replication. Although this process underlies the epigenetic inheritance, little is known about its molecular mechanisms. This mini-review is focused on Drosophila melanogaster SUppressor of UnderReplication (SUUR) protein, which is involved both in replication and chromatin maintenance in polytene tissues. Existing data suggest that it is involved in the regulation of chromatin renewal during replication. According to this model, SUUR protein moves along the chromosomes together with the replication complex. When the replication fork enters the repressed, H3K27me3- or H3K9me3-enriched, chromatin, SUUR-containing complex slows down the replisome until these histone modifications are properly placed on the newly-synthesized DNA strands. Suggested model provides an insight into the mechanism of epigenetic information inheritance. This hypothesis could be tested by further analysis of the interplay between local enrichment of repressive histone modifications and the replication fork progression rate.