Efficiency of transcription and translation of cell-free protein synthesis systems in cell-sized lipid vesicles with changing lipid composition determined by fluorescence measurements.

To develop artificial cell models that mimic living cells, cell-sized lipid vesicles encapsulating cell-free protein synthesis (CFPS) systems are useful for protein expressions or artificial gene circuits for vesicle-vesicle communications. Therefore, investigating the transcriptional and translational properties of CFPS systems in lipid vesicles is important for maximizing the synthesis and functions of proteins. Although transcription and translation using CFPS systems inside lipid vesicles are more important than that outside lipid vesicles, the former processes are not investigated by changing the lipid composition of lipid vesicles. Herein, we investigated changes in transcription and translation using CFPS systems inside giant lipid vesicles (approximately 5-20 µm in diameter) caused by changing the lipid composition of lipid vesicles containing neutral, positively, and negatively charged lipids. After incubating for 30 min, 1 h, 2 h, and 4 h, the transcriptional and translational activities in these lipid vesicles were determined by detecting the fluorescence intensities of the fluorogenic RNA aptamer on the 3'-untranslated region of mRNA (transcription) and the fluorescent protein sfCherry (translation), respectively. The results revealed that transcriptional and translational activities in a lipid vesicle containing positively charged lipids were high when the protein was synthesized using the CFPS system inside the lipid vesicle. Thus, the present study provides an experimental basis for constructing complex artificial cell models using bottom-up approaches.

Epigenetic mechanisms to propagate histone acetylation by p300/CBP.

Histone acetylation is important for the activation of gene transcription but little is known about its direct read/write mechanisms. Here, we report cryogenic electron microscopy structures in which a p300/CREB-binding protein (CBP) multidomain monomer recognizes histone H4 N-terminal tail (NT) acetylation (ac) in a nucleosome and acetylates non-H4 histone NTs within the same nucleosome. p300/CBP not only recognized H4NTac via the bromodomain pocket responsible for reading, but also interacted with the DNA minor grooves via the outside of that pocket. This directed the catalytic center of p300/CBP to one of the non-H4 histone NTs. The primary target that p300 writes by reading H4NTac was H2BNT, and H2BNTac promoted H2A-H2B dissociation from the nucleosome. We propose a model in which p300/CBP replicates histone N-terminal tail acetylation within the H3-H4 tetramer to inherit epigenetic storage, and transcribes it from the H3-H4 tetramer to the H2B-H2A dimers to activate context-dependent gene transcription through local nucleosome destabilization.

Histone H3 lysine 27 crotonylation mediates gene transcriptional repression in chromatin.

Histone lysine acylation, including acetylation and crotonylation, plays a pivotal role in gene transcription in health and diseases. However, our understanding of histone lysine acylation has been limited to gene transcriptional activation. Here, we report that histone H3 lysine 27 crotonylation (H3K27cr) directs gene transcriptional repression rather than activation. Specifically, H3K27cr in chromatin is selectively recognized by the YEATS domain of GAS41 in complex with SIN3A-HDAC1 co-repressors. Proto-oncogenic transcription factor MYC recruits GAS41/SIN3A-HDAC1 complex to repress genes in chromatin, including cell-cycle inhibitor p21. GAS41 knockout or H3K27cr-binding depletion results in p21 de-repression, cell-cycle arrest, and tumor growth inhibition in mice, explaining a causal relationship between GAS41 and MYC gene amplification and p21 downregulation in colorectal cancer. Our study suggests that H3K27 crotonylation signifies a previously unrecognized, distinct chromatin state for gene transcriptional repression in contrast to H3K27 trimethylation for transcriptional silencing and H3K27 acetylation for transcriptional activation.

H2A Ubiquitination Alters H3-tail Dynamics on Linker-DNA to Enhance H3K27 Methylation.

Polycomb repressive complex 1 (PRC1) and PRC2 are responsible for epigenetic gene regulation. PRC1 ubiquitinates histone H2A (H2Aub), which subsequently promotes PRC2 to introduce the H3 lysine 27 tri-methyl (H3K27me3) repressive chromatin mark. Although this mechanism provides a link between the two key transcriptional repressors, PRC1 and PRC2, it is unknown how histone-tail dynamics contribute to this process. Here, we have examined the effect of H2A ubiquitination and linker-DNA on H3-tail dynamics and H3K27 methylation by PRC2. In naïve nucleosomes, the H3-tail dynamically contacts linker DNA in addition to core DNA, and the linker-DNA is as important for H3K27 methylation as H2A ubiquitination. H2A ubiquitination alters contacts between the H3-tail and DNA to improve the methyltransferase activity of the PRC2-AEBP2-JARID2 complex. Collectively, our data support a model in which H2A ubiquitination by PRC1 synergizes with linker-DNA to hold H3 histone tails poised for their methylation by PRC2-AEBP2-JARID2.

Erratum: Characteristic H3 N-tail dynamics in the nucleosome core particle, nucleosome, and chromatosome.

[This corrects the article DOI: 10.1016/j.isci.2022.103937.].

Elucidation of binding preferences of YEATS domains to site-specific acetylated nucleosome core particles.

Acetylated lysine residues (Kac) in histones are recognized by epigenetic reader proteins, such as Yaf9, ENL, AF9, Taf14, and Sas5 (YEATS) domain-containing proteins. Human YEATS domains bind to the acetylated N-terminal tail of histone H3; however, their Kac-binding preferences at the level of the nucleosome are unknown. Through genetic code reprogramming, here, we established a nucleosome core particle (NCP) array containing histones that were acetylated at specific residues and used it to compare the Kac-binding preferences of human YEATS domains. We found that AF9-YEATS showed basal binding to the unmodified NCP and that it bound stronger to the NCP containing a single acetylation at one of K4, K9, K14, or K27 of H3, or to histone H4 multi-acetylated between K5 and K16. Crystal structures of AF9-YEATS in complex with an H4 peptide diacetylated either at K5/K8 or K8/K12 revealed that the aromatic cage of the YEATS domain recognized the acetylated K8 residue. Interestingly, E57 and D103 of AF9, both located outside of the aromatic cage, were shown to interact with acetylated K5 and K12 of H4, respectively, consistent with the increase in AF9-YEATS binding to the H4K8-acetylated NCP upon additional acetylation at K5 or K12. Finally, we show that a mutation of E57 to alanine in AF9-YEATS reduced the binding affinity for H4 multiacetylated NCPs containing H4K5ac. Our data suggest that the Kac-binding affinity of AF9-YEATS increases additively with the number of Kac in the histone tail.

Characteristic H3 N-tail dynamics in the nucleosome core particle, nucleosome, and chromatosome.

The nucleosome core particle (NCP) comprises a histone octamer, wrapped around by ∼146-bp DNA, while the nucleosome additionally contains linker DNA. We previously showed that, in the nucleosome, H4 N-tail acetylation enhances H3 N-tail acetylation by altering their mutual dynamics. Here, we have evaluated the roles of linker DNA and/or linker histone on H3 N-tail dynamics and acetylation by using the NCP and the chromatosome (i.e., linker histone H1.4-bound nucleosome). In contrast to the nucleosome, H3 N-tail acetylation and dynamics are greatly suppressed in the NCP regardless of H4 N-tail acetylation because the H3 N-tail is strongly bound between two DNA gyres. In the chromatosome, the asymmetric H3 N-tail adopts two conformations: one contacts two DNA gyres, as in the NCP; and one contacts linker DNA, as in the nucleosome. However, the rate of H3 N-tail acetylation is similar in the chromatosome and nucleosome. Thus, linker DNA and linker histone both regulate H3-tail dynamics and acetylation.

The N-terminal Tails of Histones H2A and H2B Adopt Two Distinct Conformations in the Nucleosome with Contact and Reduced Contact to DNA.

The nucleosome comprises two histone dimers of H2A-H2B and one histone tetramer of (H3-H4)2, wrapped around by ~145 bp of DNA. Detailed core structures of nucleosomes have been established by X-ray and cryo-EM, however, histone tails have not been visualized. Here, we have examined the dynamic structures of the H2A and H2B tails in 145-bp and 193-bp nucleosomes using NMR, and have compared them with those of the H2A and H2B tail peptides unbound and bound to DNA. Whereas the H2A C-tail adopts a single but different conformation in both nucleosomes, the N-tails of H2A and H2B adopt two distinct conformations in each nucleosome. To clarify these conformations, we conducted molecular dynamics (MD) simulations, which suggest that the H2A N-tail can locate stably in either the major or minor grooves of nucleosomal DNA. While the H2B N-tail, which sticks out between two DNA gyres in the nucleosome, was considered to adopt two different orientations, one toward the entry/exit side and one on the opposite side. Then, the H2A N-tail minor groove conformation was obtained in the H2B opposite side and the H2B N-tail interacts with DNA similarly in both sides, though more varied conformations are obtained in the entry/exit side. Collectively, the NMR findings and MD simulations suggest that the minor groove conformer of the H2A N-tail is likely to contact DNA more strongly than the major groove conformer, and the H2A N-tail reduces contact with DNA in the major groove when the H2B N-tail is located in the entry/exit side.

Quantification of the effect of site-specific histone acetylation on chromatin transcription rate.

Eukaryotic transcription is epigenetically regulated by chromatin structure and post-translational modifications (PTMs). For example, lysine acetylation in histone H4 is correlated with activation of RNA polymerase I-, II- and III-driven transcription from chromatin templates, which requires prior chromatin remodeling. However, quantitative understanding of the contribution of particular PTM states to the sequential steps of eukaryotic transcription has been hampered partially because reconstitution of a chromatin template with designed PTMs is difficult. In this study, we reconstituted a di-nucleosome with site-specifically acetylated or unmodified histone H4, which contained two copies of the Xenopus somatic 5S rRNA gene with addition of a unique sequence detectable by hybridization-assisted fluorescence correlation spectroscopy. Using a Xenopus oocyte nuclear extract, we analyzed the time course of accumulation of nascent 5S rRNA-derived transcripts generated on chromatin templates in vitro. Our mathematically described kinetic model and fitting analysis revealed that tetra-acetylation of histone H4 at K5/K8/K12/K16 increases the rate of transcriptionally competent chromatin formation ∼3-fold in comparison with the absence of acetylation. We provide a kinetic model for quantitative evaluation of the contribution of epigenetic modifications to chromatin transcription.

Acetylated histone H4 tail enhances histone H3 tail acetylation by altering their mutual dynamics in the nucleosome.

The structural unit of eukaryotic chromatin is a nucleosome, comprising two histone H2A-H2B heterodimers and one histone (H3-H4)2 tetramer, wrapped around by ∼146 bp of DNA. The N-terminal flexible histone tails stick out from the histone core and have extensive posttranslational modifications, causing epigenetic changes of chromatin. Although crystal and cryogenic electron microscopy structures of nucleosomes are available, the flexible tail structures remain elusive. Using NMR, we have examined the dynamics of histone H3 tails in nucleosomes containing unmodified and tetra-acetylated H4 tails. In unmodified nucleosome, the H3 tail adopts a dynamic equilibrium structure between DNA-contact and reduced-contact states. In acetylated H4 nucleosome, however, the H3 tail equilibrium shifts to a mainly DNA-contact state with a minor reduced-contact state. The acetylated H4 tail is dynamically released from its own DNA-contact state to a reduced-contact state, while the H3 tail DNA-contact state becomes major. Notably, H3 K14 in the acetylated H4 nucleosome is much more accessible to acetyltransferase Gcn5 relative to unmodified nucleosome, possibly due to the formation of a favorable H3 tail conformation for Gcn5. In summary, each histone tail adopts a characteristic dynamic state but regulates one other, probably creating a histone tail network even on a nucleosome.

JQ1 affects BRD2-dependent and independent transcription regulation without disrupting H4-hyperacetylated chromatin states.

The bromodomain and extra-terminal domain (BET) proteins are promising drug targets for cancer and immune diseases. However, BET inhibition effects have been studied more in the context of bromodomain-containing protein 4 (BRD4) than BRD2, and the BET protein association to histone H4-hyperacetylated chromatin is not understood at the genome-wide level. Here, we report transcription start site (TSS)-resolution integrative analyses of ChIP-seq and transcriptome profiles in human non-small cell lung cancer (NSCLC) cell line H23. We show that di-acetylation at K5 and K8 of histone H4 (H4K5acK8ac) co-localizes with H3K27ac and BRD2 in the majority of active enhancers and promoters, where BRD2 has a stronger association with H4K5acK8ac than H3K27ac. Although BET inhibition by JQ1 led to complete reduction of BRD2 binding to chromatin, only local changes of H4K5acK8ac levels were observed, suggesting that recruitment of BRD2 does not influence global histone H4 hyperacetylation levels. This finding supports a model in which recruitment of BET proteins via histone H4 hyperacetylation is predominant over hyperacetylation of histone H4 by BET protein-associated acetyltransferases. In addition, we found that a remarkable number of BRD2-bound genes, including MYC and its downstream target genes, were transcriptionally upregulated upon JQ1 treatment. Using BRD2-enriched sites and transcriptional activity analysis, we identified candidate transcription factors potentially involved in the JQ1 response in BRD2-dependent and -independent manner.

Crystal structure of human nucleosome core particle containing enzymatically introduced CpG methylation.

Cytosine methylation, predominantly of the CpG sequence in vertebrates, is one of the major epigenetic modifications crucially involved in the control of gene expression. Due to the difficulty of reconstituting site-specifically methylated nucleosomal DNA at crystallization quality, most structural analyses of CpG methylation have been performed using chemically synthesized oligonucleotides, There has been just one recent study of nucleosome core particles (NCPs) reconstituted with nonpalindromic human satellite 2-derived DNAs. Through the preparation of a 146-bp palindromic α-satellite-based nucleosomal DNA containing four CpG dinucleotide sequences and its enzymatic methylation and restriction, we reconstituted a 'symmetric' human CpG-methylated nucleosome core particle (NCP). We solved the crystal structures of the CpG-methylated and unmodified NCPs at 2.6 and 3.0 Å resolution, respectively. We observed the electron densities of two methyl groups, among the eight 5-methylcytosines introduced in the CpG-fully methylated NCP. There were no obvious structural differences between the CpG-methylated 'symmetric NCP' and the unmodified NCP. The preparation of a crystallization-grade CpG-methylated NCP provides a platform for the analysis of CpG-methyl reader and eraser proteins.

Intra- and inter-nucleosomal interactions of the histone H4 tail revealed with a human nucleosome core particle with genetically-incorporated H4 tetra-acetylation.

Post-translational modifications (PTMs) of histones, such as lysine acetylation of the N-terminal tails, play crucial roles in controlling gene expression. Due to the difficulty in reconstituting site-specifically acetylated nucleosomes with crystallization quality, structural analyses of histone acetylation are currently performed using synthesized tail peptides. Through engineering of the genetic code, translation termination, and cell-free protein synthesis, we reconstituted human H4-mono- to tetra-acetylated nucleosome core particles (NCPs), and solved the crystal structures of the H4-K5/K8/K12/K16-tetra-acetylated NCP and unmodified NCP at 2.4 Å and 2.2 Å resolutions, respectively. The structure of the H4-tetra-acetylated NCP resembled that of the unmodified NCP, and the DNA wrapped the histone octamer as precisely as in the unmodified NCP. However, the B-factors were significantly increased for the peripheral DNAs near the N-terminal tail of the intra- or inter-nucleosomal H4. In contrast, the B-factors were negligibly affected by the H4 tetra-acetylation in histone core residues, including those composing the acidic patch, and at H4-R23, which interacts with the acidic patch of the neighboring NCP. The present study revealed that the H4 tetra-acetylation impairs NCP self-association by changing the interactions of the H4 tail with DNA, and is the first demonstration of crystallization quality NCPs reconstituted with genuine PTMs.

Development of a hexahistidine-3× FLAG-tandem affinity purification method for endogenous protein complexes in Pichia pastoris.

We developed a method for efficient chromosome tagging in Pichia pastoris, using a useful tandem affinity purification (TAP) tag. The TAP tag, designated and used here as the THF tag, contains a thrombin protease cleavage site for removal of the TAP tag and a hexahistidine sequence (6× His) followed by three copies of the FLAG sequence (3× FLAG) for affinity purification. Using this method, THF-tagged RNA polymerases I, II, and III were successfully purified from P. pastoris. The method also enabled us to purify the tagged RNA polymerase II on a large scale, for its crystallization and preliminary X-ray crystallographic analysis. The method described here will be widely useful for the rapid and large-scale preparation of crystallization grade eukaryotic multi-subunit protein complexes.

Multiple site-specific installations of Nε-monomethyl-L-lysine into histone proteins by cell-based and cell-free protein synthesis.

Lysine methylation is one of the important post-translational modifications of histones, and produces an N(ε) -mono-, di-, or trimethyllysine residues. Multiple and site-specific lysine methylations of histones are essential to define epigenetic statuses and control heterochromatin formation, DNA repair, and transcription regulation. A method was previously developed to build an analogue of N(ε)-monomethyllysine, with cysteine substituting for lysine. Here, we have developed a new method of preparing histones bearing multiple N(ε)-monomethyllysine residues at specified positions. Release factor 1-knockout (RFzero) Escherichia coli cells or a cell-free system based on the RFzero cell lysate was used for protein synthesis, as in RFzero cells UAG is redefined as a sense codon for non-canonical amino acids. During protein synthesis, a tert-butyloxycarbonyl-protected N(ε)-monomethyllysine analogue is ligated to Methanosarcina mazei pyrrolysine tRNA (tRNA(Pyl)) by M. mazei pyrrolysyl-tRNA synthetase mutants, and is translationally incorporated into one or more positions specified by the UAG codon. Protecting groups on the protein are then removed with trifluoroacetic acid to generate N(ε)-monomethyllysine residues. We installed N(ε)-monomethyllysine residues at positions 4, 9, 27, 36, and/or 79 of human histone H3. Each of the N(ε)-monomethyllysine residues within the produced histone H3 was recognized by its specific antibody. Furthermore, the antibody recognized the authentic N(ε)-monomethyllysine residue at position 27 better than the N(ε)-monomethyllysine analogue built with cysteine. Mass spectrometry analyses also confirmed the lysine modifications on the produced histone H3. Thus, our method enables the installation of authentic N(ε)-monomethyllysines at multiple positions within a protein for large-scale production.

A tandem insertion vector for large-scale preparation of nucleosomal DNA.

We developed a novel nucleosome DNA template vector, pWMD01, which is optimized for the large-scale preparation of nucleosomal DNA. By using restricted digestion by SapI or EarI within its multicloning site, multiple half-nucleosome DNA units can be introduced unidirectionally into the vector at each subcloning step. Through this method, we constructed a plasmid that has 18 tandem repeats of a half-nucleosome 90-bp DNA unit containing c-Myb-binding sites in two subcloning cycles. This method enables the rapid, large-scale preparation of nucleosomal DNA with crystallization-grade quality.

Genetic-code evolution for protein synthesis with non-natural amino acids.

The genetic encoding of synthetic or "non-natural" amino acids promises to diversify the functions and structures of proteins. We applied rapid codon-reassignment for creating Escherichia coli strains unable to terminate translation at the UAG "stop" triplet, but efficiently decoding it as various tyrosine and lysine derivatives. This complete change in the UAG meaning enabled protein synthesis with these non-natural molecules at multiple defined sites, in addition to the 20 canonical amino acids. UAG was also redefined in the E. coli BL21 strain, suitable for the large-scale production of recombinant proteins, and its cell extract served the cell-free synthesis of an epigenetic protein, histone H4, fully acetylated at four specific lysine sites.

Structural implications for K5/K12-di-acetylated histone H4 recognition by the second bromodomain of BRD2.

The BET family proteins recognize acetylated chromatin through their two bromodomains, acting as transcriptional activators or tethering viral genomes to the mitotic chromosomes of their host. The structural mechanism for how the N-terminal bromodomain of human BRD2 (BRD2-BD1) deciphers the mono-acetylated status of histone H4 tail was recently reported. Here we show the crystal structure of the second bromodomain of BRD2 (BRD2-BD2) in complex with the di-acetylated histone H4 tail (H4K5ac/K12ac). To our surprise, a single K5ac/K12ac peptide interacts with two BRD2-BD2 molecules simultaneously: the K5ac residue binds to one BRD2-BD2 molecule while the K12ac residue binds to another. These results provide a structural basis for the recognition of two different patterns of the histone acetylation status by a single bromodomain.

A series of bacterial co-expression vectors with rare-cutter recognition sequences.

The bacterial co-expression method is a useful protein expression technique to reconstitute a hetero-oligomeric protein complex in vitro. However, during the plasmid subcloning for co-expression, unintended cleavage at the sequences of target cDNAs becomes more frequent as the number of DNA inserts increases. This problem also makes it difficult to change the combination of targeted proteins after preparing a certain co-expression construct. To avoid this problem, we have developed a series of bacterial co-expression vectors, in which each translation cassette can be subcloned at a set of rare-cutter restriction enzyme sites. We selected 9 different rare-cutter restriction enzymes that recognize a 7 or 8-base-pair sequence, and constructed 27 kinds of cloning vectors and 3 kinds of co-expression vectors, utilizing the rare-cutter recognition sequences as multi-cloning sites. Using this vector system, we co-expressed and co-purified a 7-subunit protein complex composed of the mammalian 26S proteasome regulatory subunits RPT1 to RPT6, and their associated factor, gankyrin. We verified the presence of all 7 subunits by western blotting, by taking advantage of the vector system in which the target proteins can be fused with a broad repertoire of epitope tags.

Purification, crystallization and preliminary X-ray diffraction of the C-terminal bromodomain from human BRD2.

BRD2 is a bromodomain-containing BET-family protein that associates with acetylated histones throughout the cell cycle. Although the tertiary structures of the bromodomains involved in histone acetyl transfer are already known, the structures of the BET-type bromodomains, which are required for tight association with acetylated chromatin, are poorly understood. Here, the expression, purification and crystallization of the C-terminal bromodomain of human BRD2 are reported. The protein was crystallized by the sitting-drop vapour-diffusion method in the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 71.78, b = 52.60, c = 32.06 A and one molecule per asymmetric unit. The crystal diffracted beyond 1.80 A resolution using synchrotron radiation.