The covalent modification of nucleosomal histones has emerged as a major determinant of chromatin structure and gene activity. To understand the interplay between various histone modifications, including acetylation and methylation, we performed a genome-wide chromatin structure analysis in a higher eukaryote. We found a binary pattern of histone modifications among euchromatic genes, with active genes being hyperacetylated for H3 and H4 and hypermethylated at Lys 4 and Lys 79 of H3, and inactive genes being hypomethylated and deacetylated at the same residues. Furthermore, the degree of modification correlates with the level of transcription, and modifications are largely restricted to transcribed regions, suggesting that their regulation is tightly linked to polymerase activity.[Keywords: Epigenetics; chromatin; histone; Drosophila; chromatin immunoprecipitation; microarray] Supplemental material is available at http://www.genesdev.org.

Whereas mono-, di- and trimethylation states of lysines on histones typically have specific functions, no specific functions have been attributed so far to the different methylation states of histone H3 Lysine 79 (H3K79) generated by Dot1. Here we show that Dot1, in contrast to other known histone methyltransferases, introduces multiple methyl groups via a nonprocessive mechanism. The kinetic mechanism implies that the H3K79 methylation states cannot be generated independently, suggesting functional redundancy. Indeed, gene silencing in yeast, which is dependent on Dot1, relied on global H3K79 methylation levels and not on one specific methylation state. Furthermore, our findings suggest that histone H2B ubiquitination affects H3K79 trimethylation by enhancing synthesis of all H3K79 methylation states. Our results suggest that multiple methylation of H3K79 leads to a binary code, which is expected to limit the possibilities for regulation by putative demethylases or binding proteins.

Post-translational modifications of histones are involved in transcript initiation and elongation. Methylation of lysine 36 of histone H3 (H3K36me) resides promoter distal at transcribed regions in Saccharomyces cerevisiae and is thought to prevent spurious initiation through recruitment of histone-deacetylase activity. Here, we report surprising complexity in distribution, regulation and readout of H3K36me in Drosophila involving two histone methyltransferases (HMTases). Dimethylation of H3K36 peaks adjacent to promoters and requires dMes-4, whereas trimethylation accumulates toward the 3 0 end of genes and relies on dHypb. Reduction of H3K36me3 is lethal in Drosophila larvae and leads to elevated levels of acetylation, specifically at lysine 16 of histone H4 (H4K16ac). In contrast, reduction of both di-and trimethylation decreases lysine 16 acetylation. Thus di-and trimethylation of H3K36 have opposite effects on H4K16 acetylation, which we propose enable dynamic changes in chromatin compaction during transcript elongation.

At yeast telomeres and silent mating-type loci, chromatin assumes a higher-order structure that represses transcription by means of the histone deacetylase Sir2 and structural proteins Sir3 and Sir4. Here, we present a fully reconstituted system to analyze SIR holocomplex binding to nucleosomal arrays. Purified Sir2-3-4 heterotrimers bind chromatin, cooperatively yielding a stable complex of homogeneous molecular weight. Remarkably, Sir2-3-4 also binds naked DNA, reflecting the strong, albeit nonspecific, DNA-binding activity of Sir4. The binding of Sir3 to nucleosomes is sensitive to histone H4 N-terminal tail removal, while that of Sir2-4 is not. Dot1-mediated methylation of histone H3K79 reduces the binding of both Sir3 and Sir2-3-4. Additionally, a byproduct of Sir2-mediated NAD hydrolysis, O-acetyl-ADP-ribose, increases the efficiency with which Sir3 and Sir2-3-4 bind nucleosomes. Thus, in small cumulative steps, each Sir protein, unmodified histone domains, and contacts with DNA contribute to the stability of the silent chromatin complex.

The dynamic behavior of proteins is critical for cellular homeostasis. However, analyzing dynamics of proteins and protein complexes in vivo has been difficult. Here we describe recombination-induced tag exchange (RITE), a genetic method that induces a permanent epitope-tag switch in the coding sequence after a hormone-induced activation of Cre recombinase. The time-controlled tag switch provides a unique ability to detect and separate old and new proteins in time and space, which opens up opportunities to investigate the dynamic behavior of proteins. We validated the technology by determining exchange of endogenous histones in chromatin by biochemical methods and by visualizing and quantifying replacement of old by new proteasomes in single cells by microscopy. RITE is widely applicable and allows probing spatiotemporal changes in protein properties by multiple methods.chromatin | histone | proteasome | protein dynamics | turnover P roteins are dynamic molecules. Their abundance is controlled by synthesis and degradation and they can be subject to posttranslational processing, modification, and demodification. In addition, most proteins are very mobile and undergo interactions with multiple other protein partners (1-4). However, little is known about the dynamics of proteins within macromolecular complexes in vivo (2, 4). Studying time-dependent changes in physical properties of proteins or protein turnover requires methods to distinguish resident (old) proteins from new proteins. Current methods that do so are usually based on fluorescent reporters or differential chemical labeling. For example, fluorescence recovery after photo bleaching relies on exchange of the old bleached protein by nonbleached proteins (1, 3, 4). Alternative methods involve time-dependent changes in fluorescence, nonspecific pulse-chase labeling of proteins with labeled amino acids, or labeling with chemical dyes that specifically bind to short tags (5-7). Although suitable for detection of proteins by microscopy or mass spectrometry, a limitation of these methods is that they do not provide a handle for biochemical analysis of old and new proteins and their complexes. To solve this problem and to eliminate the requirement for chemical labels or UV light we developed recombination-induced tag exchange (RITE), a method in which a genetic epitope tag is switched by transient induction of a site-specific recombinase. As a consequence, old and newly synthesized proteins are differentially tagged, which enables monitoring of protein dynamics by multiple techniques, as illustrated here. In contrast to inducible expression strategies (8-12), differential tagging by a time-controlled site-specific protease (13), or the labeling methods described above, RITE allows parallel detection and purification of old and new proteins under physiological conditions and over long periods of time.We used RITE to probe the stability of chromatin. Photobleaching experiments using histones tagged with fluorescent reporters suggest that chromatin is a static comp...

Tracking of ancestral histone proteins over multiple generations of genome replication in yeast reveals that old histones move along genes from 3′ toward 5′ over time, and that maternal histones move up to around 400 bp during genomic replication.

DNA replication fork stalling poses a major threat to genome stability. This is counteracted in part by the intra-S phase checkpoint, which stabilizes arrested replication machinery, prevents cell-cycle progression and promotes DNA repair. The checkpoint kinase Mec1/ATR and RecQ helicase Sgs1/BLM contribute synergistically to fork maintenance on hydroxyurea (HU). Both enzymes interact with replication protein A (RPA). We identified and deleted the major interaction sites on Sgs1 for Rpa70, generating a mutant called sgs1-r1. In contrast to a helicase-dead mutant of Sgs1, sgs1-r1 did not significantly reduce recovery of DNA polymerase α at HU-arrested replication forks. However, the Sgs1 R1 domain is a target of Mec1 kinase, deletion of which compromises Rad53 activation on HU. Full activation of Rad53 is achieved through phosphorylation of the Sgs1 R1 domain by Mec1, which promotes Sgs1 binding to the FHA1 domain of Rad53 with high affinity. We propose that the recruitment of Rad53 by phosphorylated Sgs1 promotes the replication checkpoint response on HU. Loss of the R1 domain increases lethality selectively in cells lacking Mus81, Slx4, Slx5 or Slx8.

SUMMARY Mec1-Ddc2 (ATR-ATRIP) controls the DNA damage checkpoint and shows differential cell-cycle regulation in yeast. To find regulators of Mec1-Ddc2, we exploited a mec1 mutant that retains catalytic activity in G2 and recruitment to stalled replication forks, but which is compromised for the intra-S phase checkpoint. Two screens, one for spontaneous survivors and an E-MAP screen for synthetic growth effects, identified loss of PP4 phosphatase, pph3Δ and psy2Δ, as the strongest suppressors of mec1-100 lethality on HU. Restored Rad53 phosphorylation accounts for part, but not all, of the pph3Δ-mediated survival. Phosphoproteomic analysis confirmed that 94% of the mec1-100-compromised targets on HU are PP4 regulated, including a phosphoacceptor site within Mec1 itself, mutation of which confers damage sensitivity. Physical interaction between Pph3 and Mec1, mediated by cofactors Psy2 and Ddc2, is shown biochemically and through FRET in subnuclear repair foci. This establishes a physical and functional Mec1-PP4 unit for regulating the checkpoint response.