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J. Biol. Chem., Vol. 282, Issue 19, 14065-14072, May 11, 2007

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HULC, a Histone H2B Ubiquitinating Complex, Modulates Heterochromatin Independent of Histone Methylation in Fission Yeast^*

From the Laboratory of Biochemistry and Molecular Biology, NCI, National Institutes of Health, Bethesda, Maryland 20892

Heterochromatin in fission yeast is targeted dynamically by opposing chromatin-modifying activities capable of alleviating or promoting transcriptional gene silencing. In this study, we report the biochemical and genetic characterization of a ubiquitin-conjugating enzyme Rhp6 (a homolog of budding yeast Rad6), which has been shown to negatively affect stability of heterochromatic structures. We show that Rhp6 is a component of the multisubunit protein complex (termed HULC) that also contains two RING finger proteins Rfp1 and Rfp2, sharing homology with budding yeast Bre1 protein and a unique serine-rich protein Shf1. HULC is required for ubiquitination of histone H2B at lysine 119 (H2B-K119), and it localizes to heterochromatic sequences. Moreover, our analyses suggest that Rhp6-induced changes in heterochromatic silencing are mediated predominantly through H2B ubiquitination (ubH2B), and they correlate with increased RNA polymerase II levels at repeat elements embedded within heterochromatin domains. Interestingly, heterochromatic derepression caused by Rhp6 occurs independently of the involvement of HULC subunits and ubH2B in methylation of histone H3 at lysine 4 (H3K4me). These analyses implicate ubH2B in modulation of heterochromatin, which has important implications for dynamics and many functions associated with heterochromatic structures.

Received for publication, January 10, 2007 , and in revised form, March 9, 2007.

^* This work was supported by the Intramural Research Program of the National Institutes of Health, National Cancer Institute, Center for Cancer Research. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. 1–3.

¹ To whom correspondence should be addressed. Tel.: 301-594-6389; Fax: 301-435-3697; E-mail: grewals{at}mail.nih.gov.

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