The organization of chromatin into higher-order structures influences chromosome function and epigenetic gene regulation. Higher-order chromatin has been proposed to be nucleated by the covalent modification of histone tails and the subsequent establishment of chromosomal subdomains by non-histone modifier factors. Here we show that human SUV39H1 and murine Suv39h1—mammalian homologues of Drosophila Su(var)3-9 and of Schizosaccharomyces pombe clr4—encode histone H3-specific methyltransferases that selectively methylate lysine 9 of the amino terminus of histone H3 in vitro. We mapped the catalytic motif to the evolutionarily conserved SET domain, which requires adjacent cysteine-rich regions to confer histone methyltransferase activity. Methylation of lysine 9 interferes with phosphorylation of serine 10, but is also influenced by pre-existing modifications in the amino terminus of H3. In vivo, deregulated SUV39H1 or disrupted Suv39h activity modulate H3 serine 10 phosphorylation in native chromatin and induce aberrant mitotic divisions. Our data reveal a functional interdependence of site-specific H3 tail modifications and suggest a dynamic mechanism for the regulation of higher-order chromatin.

Regulation of chromatin structure by site-specific histone H3 methyltransferases

https://mcb.berkeley.edu/courses/mcb230/WEB/PAPERS/jenuwein%20cell%20suv39%20genome%20stability%20copy.pdf

Loss of the Suv39h histone methyltransferases impairs mammalian heterochromatin and genome stability.

Histone methylation versus histone acetylation: new insights into epigenetic regulation.

Su(var)3–9 is a dominant modifier of heterochromatin-induced gene silencing. Like its mammalian and Schizosaccharomyces pombe homologues, Su(var) 3–9 encodes a histone methyltransferase (HMTase), which selectively methylates histone H3 at lysine 9 (H3-K9). In Su(var)3–9 null mutants, H3-K9 methylation at chromocentre heterochromatin is strongly reduced, indicating that SU(VAR)3–9 is the major heterochromatin-specific HMTase in Drosophila. SU (VAR)3–9 interacts with the heterochromatin-associated HP1 protein and with another silencing factor, SU(VAR)3–7. Notably, SU(VAR)3–9–HP1 interaction is interdependent and governs distinct localization patterns of both proteins. In Su(var)3–9 null mutants, concentration of HP1 at the chromocentre is nearly lost without affecting HP1 accumulation at the fourth chromosome. By contrast, in HP1 null mutants SU(VAR)3–9 is no longer restricted at heterochromatin but broadly dispersed across the chromosomes. Despite this interdependence, Su(var)3–9 dominates the PEV modifier effects of HP1 and Su(var)3–7 and is also epistatic to the Y chromosome effect on PEV. Finally, the human SUV39H1 gene is able to partially rescue Su(var)3–9 silencing defects. Together, these data indicate a central role for the SU(VAR)3–9 HMTase in heterochromatin-induced gene silencing in Drosophila.

/pdf/central-role-of-drosophila-su-var-3-9-in-histone-h3-k9-3n8l504arb.pdf

Central role of Drosophila SU(VAR)3-9 in histone H3-K9 methylation and heterochromatic gene silencing.

Heterochromatin is both necessary for the expression of heterochromatic genes and inhibitory for the expression of euchromatic genes These two properties of heterochromatin have been elucidated from the study of chromosome rearrange­ ments that induce position effect variegation (PEV) in Drosophila melanogaster Novel euchromatin-heteroc hromatin can affect the expression of euchromatic and heterochromatic genes located several megabases away, distin­ guishing higher order chromatin structure from most other regulatory mecha­ nis ms Studies of PEV promis e insights into the basis for heterochromatin formation and the role of higher order chromatin and chromosome structure in gene regulation We eva luate the models and experimental data that address the mechanisms of PEV in different cell types, the potential functions of modifiers of PEV, and the relationship of PEV to other phenomena associated with vari­ egated gene expression in Drosophila

Heterochromatin and gene expression in Drosophila.

Chromosome rearrangements which place euchromatic genes adjacent to a heterochromatic breakpoint frequently result in gene repression (position-effect variegation). This repression is thought to reflect the spreading of a heterochromatic structure into neighboring euchromatin. Two allelic dominant suppressors of position-effect variegation were found to contain mutations within the gene encoding the heterochromatin-specific chromosomal protein HP-1. The site of mutation for each allele is given: one converts Lys169 into a nonsense (ochre) codon, while the other is a frameshift after Ser10. In flies heterozygous for one of the mutant alleles (Su(var)2-504), a truncated HP-1 protein was detectable by Western blot analysis. An HP-1 minigene, consisting of HP-1 cDNA under the control of an Hsp70 heat-inducible promoter, was transduced into flies by P element-mediated germ line transformation. Heat-shock driven expression of this minigene results in elevated HP-1 protein level and enhancement of position-effect variegation. Levels of variegating gene expression thus appear to depend upon the level of expression of a heterochromatin-specific protein. The implications of these observations for mechanism of heterochromatic position effects and heterochromatin function are discussed.

https://www.genetics.org/content/genetics/131/2/345.full.pdf

G D Morris

Papers

The heterochromatin-associated protein HP-1 is an essential protein in Drosophila with dosage-dependent effects on position-effect variegation.