HP1 Is Essential for DNA Methylation in Neurospora

Abstract

Methylation of cytosines silences transposable elements and selected cellular genes in mammals, plants, and some fungi. Recent findings have revealed mechanistic connections between DNA methylation and features of specialized condensed chromatin, "heterochromatin." In Neurospora crassa, DNA methylation depends on trimethylation of Lys9 in histone H3 by DIM-5. Heterochromatin protein HP1 binds methylated Lys9 in vitro. We therefore investigated the possibility that a Neurospora HP1 homolog reads the methyl-Lys9 mark to signal DNA methylation. We identified an HP1 homolog and showed that it is essential for DNA methylation, is localized to heterochromatic foci, and that this localization is dependent on the catalytic activity of DIM-5. We conclude that HP1 serves as an adaptor between methylated H3 Lys9 and the DNA methylation machinery. Unlike mutants that lack DNA methyltransferase, mutants with defects in the HP1 gene hpo exhibit severe growth defects, suggesting that HP1 is required for processes besides DNA methylation.

DNA methylation and the formation of heterochromatin in Neurospora crassa

“…In contrast to the results in fission yeast and flies, inactivation of the hpo gene, which encodes the HP1 homologue of Neurospora, showed very little alteration in H3K9me3 (Lewis et al, 2009). However, hpo mutants show a complete loss of DNA methylation in vegetative tissue, supporting the idea that this protein is required for reading the H3K9me3 mark and relaying this information to the DNA methylation machinery (Table 1) (Freitag et al, 2004b). Consistent with this, HP1 was found to generally co-localize with H3K9me3 along Linkage Group V11 in Neurospora (Lewis et al, 2009).…”

Neurospora crassa, a Model System for Epigenetics Research

“…In particular, knowledge that HP1, a protein first identified in Drosophila, binds H3K9me3 in vitro (discussed in Elgin and Reuter 2013), motivated a search for an HP1 homolog in Neurospora. A likely homolog was found and its involvement in DNA methylation was tested by gene disruption (Freitag et al 2004a). The gene, named hpo (HP one), was indeed essential for DNA methylation.…”

“…Neurospora has homologs of a variety of genes implicated in RNAi (Galagan et al 2003;Borkovich et al 2004). Studies of mutants with null mutations in all three RNA-dependent RNA polymerase (RdRP) genes, in both Dicer genes, or in other presumptive RNAi genes revealed no evidence that RNAi is involved in methylation of H3K9, heterochromatin formation, or DNA methylation in Neurospora (Chicas et al 2004;Freitag et al 2004a;Lewis et al 2009). However, as discussed in Sections 6 and 7, the Neurospora RNAi genes are involved in at least two other silencing mechanisms with epigenetic aspects, quelling, and meiotic silencing.…”

Gradual development of a genome‐wide H3‐K9 trimethylation pattern in paternally derived pig pronucleus

“…has been observed in diverse biological systems (Tamaru and Selker, 2001;Fahrner et al, 2002;Jackson et al, 2002;Fuks et al, 2003;Lehnertz et al, 2003;Tariq et al, 2003;Freitag et al, 2004). Therefore, the different H3-m 3 K9 states in the two species prompted us to investigate DNA methylation states in sperm-derived chromatins.…”