Drosophila ISWI Regulates the Association of Histone H1 With Interphase Chromosomesin Vivo

Abstract

Although tremendous progress has been made toward identifying factors that regulate nucleosome structure and positioning, the mechanisms that regulate higher-order chromatin structure remain poorly understood. Recent studies suggest that the ISWI chromatin-remodeling factor plays a key role in this process by promoting the assembly of chromatin containing histone H1. To test this hypothesis, we investigated the function of H1 in Drosophila. The association of H1 with salivary gland polytene chromosomes is regulated by a dynamic, ATP-dependent process. Reducing cellular ATP levels triggers the dissociation of H1 from polytene chromosomes and causes chromosome defects similar to those resulting from the loss of ISWI function. H1 knockdown causes even more severe defects in chromosome structure and a reduction in nucleosome repeat length, presumably due to the failure to incorporate H1 during replication-dependent chromatin assembly. Our findings suggest that ISWI regulates higher-order chromatin structure by modulating the interaction of H1 with interphase chromosomes. T HE packaging of DNA into chromatin is critical for the organization and regulation of eukaryotic genes. The basic unit of chromatin structure-the nucleosome--can be packaged in 30-nm fibers and increasingly compact structures. Higher-order chromatin structure influences many aspects of gene expression, including transcription factor binding, enhancer-promoter interactions, and the organization of chromatin into functional domains. Histone H1 and related linker histones are important determinants of higher-order chromatin structure. These abundant, basic proteins share a common structure consisting of a globular winged helix DNA-binding domain flanked by a short N-terminal segment and a C-terminal domain of 100 amino acids (Brown 2003). The winged helix domain of H1 binds the nucleosome near the site of DNA entry and exit; the flanking domains interact with core and linker DNA to promote the formation and packaging of 30-nm fibers in vitro (Robinson and Rhodes 2006;Maier et al. 2008).

Dosage Compensation inDrosophila

“…Nevertheless, high-resolution mapping of nucleosomes in Iswi mutant males and females indicates that the chromosome condensation defects that they show, especially of the X chromosome in males, are not correlated with global ISWI-dependent nucleosome spacing changes (Sala et al 2011). A more convincing explanation is provided by the observation that ISWI facilitates the association of linker histone H1 and that a reduction of the H1 leads to a chromosomal phenotype similar to the one described above for ISWI loss of function (Siriaco et al 2009). Somewhat surprisingly, loss of function of the RSF complex does not appear to alter chromosome structure (Hanai et al 2008).…”

The Nucleosome Remodeling Factor ISWI Functionally Interacts With an Evolutionarily Conserved Network of Cellular Factors

“…In Drosophila, loss of ISWI function causes global transcription defects and leads to dramatic alterations in higher-order chromatin structure, including the apparent decondensation of both mitotic and interphase chromosomes (Deuring et al 2000;Corona et al 2007). Recent findings indicate that ISWI controls chromosome compaction in vivo, in part through its ability to promote the association of the linker histone H1 with chromatin Siriaco et al 2009). …”

A Novel Approach for Studying Histone H1 Function in Vivo

“…This conserved linker histone contains a winged helix domain that binds the nucleosome near the site of DNA entry and exit; the regions flanking this domain interact with core histones and linker DNA to package nucleosome arrays into 30-nm fibers in vitro (Harshman et al 2013). Consistent with its biochemical properties, the loss of histone H1 function leads to chromosome decondensation, defects in gene expression, genomic instability, and lethality (Godde and Ura 2008;Happel and Doenecke 2009;Lu et al 2009;Siriaco et al 2009;Vujatovic et al 2012;Lu et al 2013). These findings have stimulated great interest in the mechanisms that regulate the expression, assembly, and function of histone H1.…”

“…The expression of histone H1 must be tightly regulated, as even modest changes in the level of histone H1 can have dramatic effects on nucleosome repeat length, global nucleosome density, and chromatin compaction (Blank and Becker 1995;Woodcock et al 2006;Routh et al 2008;Siriaco et al 2009). In budding yeast, core histones are subject to negative autoregulation (Gunjan et al 2006;Eriksson et al 2012).…”

“…By contrast, even the partial knockdown of histone H1 levels by RNAi blocks development and causes dramatic defects in polytene chromosome structure (Lu et al 2009;Siriaco et al 2009). These findings indicate that histone H1-GFP is functionally equivalent to the endogenous histone H1 protein and can be used to study histone H1 function in vivo.…”