HP1a Recruitment to Promoters Is Independent of H3K9 Methylation in Drosophila melanogaster

Margarida L A Figueiredo,Per Stenberg,Jan Larsson,Philge Philip,Rob A H White

doi:10.1371/journal.pgen.1003061

Margarida L A Figueiredo, Per Stenberg + Show 3 more

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https://doi.org/10.1371/journal.pgen.1003061

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Journal: PLoS Genetics	Publication Date: Nov 15, 2012
Citations: 111	License type: CC BY 4.0

Affiliation: Umeå University

Abstract

Heterochromatin protein 1 (HP1) proteins, recognized readers of the heterochromatin mark methylation of histone H3 lysine 9 (H3K9me), are important regulators of heterochromatin-mediated gene silencing and chromosome structure. In Drosophila melanogaster three histone lysine methyl transferases (HKMTs) are associated with the methylation of H3K9: Su(var)3-9, Setdb1, and G9a. To probe the dependence of HP1a binding on H3K9me, its dependence on these three HKMTs, and the division of labor between the HKMTs, we have examined correlations between HP1a binding and H3K9me patterns in wild type and null mutants of these HKMTs. We show here that Su(var)3-9 controls H3K9me-dependent binding of HP1a in pericentromeric regions, while Setdb1 controls it in cytological region 2L:31 and (together with POF) in chromosome 4. HP1a binds to the promoters and within bodies of active genes in these three regions. More importantly, however, HP1a binding at promoters of active genes is independent of H3K9me and POF. Rather, it is associated with heterochromatin protein 2 (HP2) and open chromatin. Our results support a hypothesis in which HP1a nucleates with high affinity independently of H3K9me in promoters of active genes and then spreads via H3K9 methylation and transient looping contacts with those H3K9me target sites.

Highlights

Genomic DNA in eukaryotes is organized into chromatin, which has historically been divided into two distinct forms, euchromatin and heterochromatin, based on histological staining patterns
We conclude that HP1a binding on the 4th chromosome and region 2L:31 depends on Setdb1, HP1a binding in pericentromeric regions depends on Su(var)3-9 while we found no clear dependence of its binding on G9a in our experiments
The results show that H3K9me2 was strongly reduced in region 2L:31 and the 4th chromosome, while virtually all H3K9me3 was lost in region 2L:31 and strongly reduced on chromosome 4 in Setdb1 mutants (Figure 2)

Summary

Introduction

Genomic DNA in eukaryotes is organized into chromatin, which has historically been divided into two distinct forms, euchromatin and heterochromatin, based on histological staining patterns. Three H3K9-specific methyl transferases (HKMTs) have been described in Drosophila: Su(var), Setdb and G9a. Su(var) and G9a, in contrast, are not essential for viability and their role during development appears to be less specific [6,7,8,9]. Chromosome staining experiments have shown that levels of pericentric H3K9me are reduced, but the enrichment of H3K9me on the 4th chromosome remains unaltered in Su(var) mutants [8,10,11]. The roles and specificity of G9a remains unclear, as does the redundancy of the three HKMTs [6,7,9]

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