p53 is a potent tumor suppressor, whose biological effects are largely due to its function as a transcriptional regulator. Here we report that, in addition to regulating the expression of hundreds of protein-coding genes, p53 also modulates the levels of microRNAs (miRNAs). Specifically, p53 can induce expression of microRNA-34a (miR-34a) in cultured cells as well as in irradiated mice, by binding to a perfect p53 binding site located within the gene that gives rise to miR-34a. Processing of the primary transcript into mature miR-34a involves the excision of a 30 kb intron. Notably, inactivation of miR-34a strongly attenuates p53-mediated apoptosis in cells exposed to genotoxic stress, whereas overexpression of miR-34a mildly increases apoptosis. Hence, miR-34a is a direct proapoptotic transcriptional target of p53 that can mediate some of p53's biological effects. Perturbation of miR-34a expression, as occurs in some human cancers, may thus contribute to tumorigenesis by attenuating p53-dependent apoptosis.

Histone monoubiquitylation is implicated in critical regulatory processes. We explored the roles of histone H2B ubiquitylation in human cells by reducing the expression of hBRE1/RNF20, the major H2B-specific E3 ubiquitin ligase. While H2B ubiquitylation is broadly associated with transcribed genes, only a subset of genes was transcriptionally affected by RNF20 depletion and abrogation of H2B ubiquitylation. Gene expression dependent on RNF20 includes histones H2A and H2B and the p53 tumor suppressor. In contrast, RNF20 suppresses the expression of several proto-oncogenes, which reside preferentially in closed chromatin and are modestly transcribed despite bearing marks usually associated with high transcription rates. Remarkably, RNF20 depletion augmented the transcriptional effects of epidermal growth factor (EGF), increased cell migration, and elicited transformation and tumorigenesis. Furthermore, frequent RNF20 promoter hypermethylation was observed in tumors. RNF20 may thus be a putative tumor suppressor, acting through selective regulation of a distinct subset of genes.[Keywords: RNF20; BRE1; H2B ubiquitylation; tumor suppressor; transcription] Supplemental material is available at http://www.genesdev.org. Received June 6, 2008; revised version accepted August 12, 2008. Eukaryotic DNA is packaged into a chromatin structure of repeating nucleosomes consisting of DNA wrapped around an octamer of core histone proteins (H2A, H2B, H3, and H4). The histone tails, which protrude from the nucleosome, are subjected to a multitude of covalent modifications believed to play a vital role in chromatin remodeling and transcriptional regulation (Jenuwein and Allis 2001;Berger 2007;. One such modification is histone H2B monoubiquitylation. In the yeast S. cerevisiae this process is mediated by the E3 ligase BRE1 (Hwang et al. 2003). In mammals, the hBRE1(RNF20)/RNF40 complex was shown to function as the relevant E3 ligase (Kim et al. 2005;Zhu et al. 2005). In yeast, transcription of several inducible genes is impaired in the absence of ubiquitylated H2B (H2Bub) (Kao et al. 2004). Increased levels of H2Bub occur on the GAL1 core promoter and throughout the transcribed region upon transcriptional activation, with both ubiquitylation and deubiquitylation being required for optimal transcription (Henry et al. 2003;Xiao et al. 2005). Moreover, H2B monoubiquitylation was shown to lead to H3 methylation on Lys 4 and Lys 79, considered marks of actively transcribed genes (Briggs et al. 2002;Sun and Allis 2002). Yet, a recent study suggests that H2B ubiquitylation in S. pombe controls transcriptional elongation by RNA polymerase II (Pol II) independently of H3 methylation (Tanny et al. 2007).Along with the studies linking H2Bub positively with active transcription, other reports suggest a link between

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