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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2006.04.029 Byline: Rushad Pavri (1), Bing Zhu (1), Guohong Li (1), Patrick Trojer (1), Subhrangsu Mandal (1), Ali Shilatifard (2), Danny Reinberg (1) Abstract: Over the past years, a large number of histone posttranslational modifications have been described, some of which function to attain a repressed chromatin structure, while others facilitate activation by allowing access of regulators to DNA. Histone H2B monoubiquitination is a mark associated with transcriptional activity. Using a highly reconstituted chromatin-transcription system incorporating the inducible RAR[beta]2 promoter, we find that the establishment of H2B monoubiquitination by RNF20/40 and UbcH6 is dependent on the transcription elongation regulator complex PAF, the histone chaperone FACT, and transcription. H2B monoubiquitination facilitates FACT function, thereby stimulating transcript elongation and the generation of longer transcripts. These in vitro analyses and corroborating in vivo experiments demonstrate that elongation by RNA polymerase II through the nucleosomal barrier is minimally dependent upon (1) FACT and (2) the recruitment of PAF and the H2B monoubiquitination machinery. Author Affiliation: (1) Howard Hughes Medical Institute, Department of Biochemistry, Division of Nucleic Acids Enzymology, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, 683 Hoes Lane, Piscataway, NJ 08854, USA (2) Department of Biochemistry, Saint Louis University School of Medicine, 1402 South Grand Boulevard, Saint Louis, MI 63104, USA Article History: Received 9 February 2006; Revised 23 March 2006; Accepted 20 April 2006 Article Note: (miscellaneous) Published: May 18, 2006

Activation-induced cytidine deaminase (AID) initiates class switch recombination (CSR) and somatic hypermutation (SHM) by de-aminating cytosine residues in immunoglobulin genes (Igh, Igc, and Igλ). At a lower frequency, AID also causes collateral DNA damage at non-lg loci, including genes that are rearranged or mutated in B-cell lymphoma. Precisely how AID is recruited to these off-target sites is not entirely understood. To gain further insight into how AID selects its targets, we compared AID-mediated translocations in two different cell types, B cells and mouse embryonic fibroblasts (MEFs). AID targets a distinct set of hotspots in the two cell types. In both cases, hotspots are concentrated in highly transcribed but stalled genes. However, transcription alone is insufficient to recruit AID activity. Comparison of genes similarly transcribed in B cells and MEFs but targeted in only one of the two cell types reveals a common set of epigenetic features associated with AID recruitment in both cells. AID target genes are enriched in chromatin modifications associated with active enhancers (such as H3K27Ac) and marks of active transcription (such as H3K36me3) in both fibroblasts and B cells, indicating that these features are universal mediators of AID recruitment. [ABSTRACT FROM AUTHOR]

The six subunit Origin Recognition Complex (ORC) loads excess MCM2-7 on chromosomes to promote initiation of DNA replication and is believed to be important for origin specification. Mapping of origins in cancer cell lines engineered to delete three of the subunits, ORC1 , ORC2 or ORC5 shows that specific origins are still used and are mostly at the same sites in the genome as in wild type cells. The few thousand origins that were up-regulated in the absence of ORC suggest that GC/TA skewness and simple repeat sequences facilitate, but are not essential for, origin selection in the absence of the six-subunit ORC. Despite the lack of ORC, excess MCM2-7 is still loaded at comparable rates in G1 phase to license dormant origins and is also repeatedly loaded in the same S phase to permit re-replication. Thus, origin specification and excess MCM2-7 loading on origins do not require the six-subunit ORC in human cancer cell lines.