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

Abstract

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 cycledependent 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.

Emerging roles of linker histones in regulating chromatin structure and function

“…4b). Additional evidence suggested further SUUR-independent roles for H1 in replication 103 . Strikingly, H1 was shown to exhibit a very dynamic temporal distribution pattern in polytene chromosomes during the endocycle.…”

“…5a). As described above, D. melanogaster H1 is required for tethering the H3K9 HMT Su(var)3-9 and the SNF2-like ATPase SUUR to their cognate loci in larval polytene chromosomes 84,103 . Binding of specific murine H1 subtypes to DNMT1 and DNMT3B helps to mediate their recruitment to ICRs, DNA methylation and gene repression 86 .…”

“…melanogaster undergoing endoreplication 103 . In this context, H1 was found to be an upstream effector of protein suppressor of underreplication (SUUR).…”

“…The pre-replicative marking of intercalary heterochromatin with H1 precedes the subsequent deposition of SUUR, which occurs in these domains only after a delay of several hours. Thus, early in the endo-S phase, H1 may act directly, without SUUR, to suppress the activation of late origins of replication, thereby inhibiting DNA endoreplication of these loci until later in endo-S 103 . Although the mechanism of replication-independent H1 loading into intercalary heterochromatin is not understood, it is possible that the highly restricted distribution of H1 in early endo-S is established through epigenetic mechanisms, such as H1 tethering to the H3K27me3 mark 90 , which is highly abundant in intercalary heterochromatin 105 .…”

“…5b). Whereas the globular domain of H1 is required for its deposition into chromatin and for physical interactions with HP1 (REFS 36,137), different regions of the C-terminal domain bind to linker DNA 10 , promote oligonucleosome condensation 138,139 and mediate interactions of H1 with DNMTs, STAT, Su(var)3-9 and SUUR 86,103,132,137 .…”