Chiral-at-metal bis-cyclometalated iridium(III) complexes are introduced as a new class of chiral catalysts for the reaction of epoxides with CO
to form cyclic carbonates under conditions of kinetic ...resolution. Reactions are typically performed at room temperature in the presence of 1 mol % of iridium catalyst and 1.5 mol % of tetraethylammonium bromide as the nucleophilic cocatalyst to provide selectivity factors of up to 16.6. A variety of monosubstituted epoxides, including styrene epoxide, epoxides with aliphatic side chains, glycidyl ethers, and a glycidyl ester, are found to be suitable substrates. No polymerization side reaction is observed for any of the investigated substrates.
The rDNA clusters and flanking sequences on human chromosomes 13, 14, 15, 21 and 22 represent large gaps in the current genomic assembly. The organization and the degree of divergence of the human ...rDNA units within an individual nucleolar organizer region (NOR) are only partially known. To address this lacuna, we previously applied transformation-associated recombination (TAR) cloning to isolate individual rDNA units from chromosome 21. That approach revealed an unexpectedly high level of heterogeneity in human rDNA, raising the possibility of corresponding variations in ribosome dynamics. We have now applied the same strategy to analyze an entire rDNA array end-to-end from a copy of chromosome 22. Sequencing of TAR isolates provided the entire NOR sequence, including proximal and distal junctions that may be involved in nucleolar function. Comparison of the newly sequenced rDNAs to reference sequence for chromosomes 22 and 21 revealed variants that are shared in human rDNA in individuals from different ethnic groups, many of them at high frequency. Analysis infers comparable intra- and inter-individual divergence of rDNA units on the same and different chromosomes, supporting the concerted evolution of rDNA units. The results provide a route to investigate further the role of rDNA variation in nucleolar formation and in the empirical associations of nucleoli with pathology.
The kinetochore is responsible for accurate chromosome segregation. However, the mechanism by which kinetochores assemble and are maintained remains unclear. Here we report that de novo CENP‐A ...assembly and kinetochore formation on human centromeric alphoid DNA arrays is regulated by a histone H3K9 acetyl/methyl balance. Tethering of histone acetyltransferases (HATs) to alphoid DNA arrays breaks a cell type‐specific barrier for de novo stable CENP‐A assembly and induces assembly of other kinetochore proteins at the ectopic alphoid site. Similar results are obtained following tethering of CENP‐A deposition factors hMis18α or HJURP. HAT tethering bypasses the need for hMis18α, but HJURP is still required for de novo kinetochore assembly. In contrast, H3K9 methylation following tethering of H3K9 tri‐methylase (Suv39h1) to the array prevents de novo CENP‐A assembly and kinetochore formation. CENP‐A arrays assembled de novo by this mechanism can form human artificial chromosomes (HACs) that are propagated indefinitely in human cells.
Establishment of Human Artificial Chromosomes (HACs) depends on an interplay of H3 lysine 9 modifications at centromeres, providing insights into the pathways that control incorporation of the kinetochore‐specificing histone H3 variant CENP‐A.
We report here a practical protocol for the asymmetric synthesis of amino acids (AAs) with a CF3‐containing 3,2’‐pyrrolidinyl spirooxindole skeleton with three defined carbon stereocenters via a ...sequential Michael/Mannich 3+2‐cycloaddition reaction. The coupling of a robust and stereochemically stable chiral dehydroalanine Ni(II) complex with various N‐2,2,2‐trifluoroethylisatin ketimines in the presence of triethylamine afforded a library of single diastereomeric complexes with a 3,2’‐pyrrolidinyl spirooxindole moiety in 36–71% yields. In particular, the change of base to LiOH allowed to obtain predominantly the Michael addition product in 76% yield. Finally, the decomposition of the obtained Ni(II) complexes with 3 N HCl provided the target complex AA with a 3,2’‐pyrrolidinyl spirooxindole core and (2S,4R)‐2,4‐diamino‐5,5,5‐trifluoropentanoic acid – an (S)‐norvaline derivative, together with easy recovery of the chiral auxiliary ligand for the synthesis of the starting dehydroalanine complex.
Abstract
Despite the key role of the human ribosome in protein biosynthesis, little is known about the extent of sequence variation in ribosomal DNA (rDNA) or its pre-rRNA and rRNA products. We ...recovered ribosomal DNA segments from a single human chromosome 21 using transformation-associated recombination (TAR) cloning in yeast. Accurate long-read sequencing of 13 isolates covering ∼0.82 Mb of the chromosome 21 rDNA complement revealed substantial variation among tandem repeat rDNA copies, several palindromic structures and potential errors in the previous reference sequence. These clones revealed 101 variant positions in the 45S transcription unit and 235 in the intergenic spacer sequence. Approximately 60% of the 45S variants were confirmed in independent whole-genome or RNA-seq data, with 47 of these further observed in mature 18S/28S rRNA sequences. TAR cloning and long-read sequencing enabled the accurate reconstruction of multiple rDNA units and a new, high-quality 44 838 bp rDNA reference sequence, which we have annotated with variants detected from chromosome 21 of a single individual. The large number of variants observed reveal heterogeneity in human rDNA, opening up the possibility of corresponding variations in ribosome dynamics.
CENP-B binds to CENP-B boxes on centromeric satellite DNAs (known as alphoid DNA in humans). CENP-B maintains kinetochore function through interactions with CENP-A nucleosomes and CENP-C. CENP-B ...binding to transfected alphoid DNA can induce
CENP-A assembly, functional centromere and kinetochore formation, and subsequent human artificial chromosome (HAC) formation. Furthermore, CENP-B also facilitates H3K9 (histone H3 lysine 9) trimethylation on alphoid DNA, mediated by Suv39h1, at ectopic alphoid DNA integration sites. Excessive heterochromatin invasion into centromere chromatin suppresses CENP-A assembly. It is unclear how CENP-B controls such different chromatin states. Here, we show that the CENP-B acidic domain recruits histone chaperones and many chromatin modifiers, including the H3K36 methylase ASH1L, as well as the heterochromatin components Suv39h1 and HP1 (HP1α, β and γ, also known as CBX5, CBX1 and CBX3, respectively). ASH1L facilitates the formation of open chromatin competent for CENP-A assembly on alphoid DNA. These results indicate that CENP-B is a nexus for histone modifiers that alternatively promote or suppress CENP-A assembly by mutually exclusive mechanisms. Besides the DNA-binding domain, the CENP-B acidic domain also facilitates CENP-A assembly
on transfected alphoid DNA. CENP-B therefore balances CENP-A assembly and heterochromatin formation on satellite DNA.
Aliphatic artificial α-amino acids (α-AAs) have attracted great interest in biochemistry and pharmacy. In this context, we developed a promising practical protocol for the asymmetric synthesis of ...these α-AAs through the selective and efficient intermolecular cross-electrophile coupling of Belokon's chiral dehydroalanine Ni(
ii
) complex with different alkyl and perfluoroalkyl iodides mediated by a dual Zn/Cu system. The reaction afforded diastereomeric complexes with dr up to 21.3 : 1 in 24-95% yields (19 examples). Exemplarily, three enantiomerically pure aliphatic α-AAs were obtained through acidic decomposition of (
S
,
S
)-diastereomers of Ni(
ii
) complexes. Importantly, the chiral auxiliary ligand (
S
)-
BPB
((
S
)-2-(
N
-benzylprolyl)aminobenzophenone) was easily recycled by simple filtration after acidic complex decomposition and reused for the synthesis of the initial dehydroalanine Ni(
ii
) complex.
A general protocol for the asymmetric synthesis of artificial aliphatic and perfluoroalkylated α-amino acids
via
Luche's cross-electrophile coupling reaction promoted by a dual Zn/Cu system is reported.
Completion of the human genome sequence and recent advances in engineering technologies have enabled an unprecedented level of understanding of DNA variations and their contribution to human diseases ...and cellular functions. However, in some cases, long-read sequencing technologies do not allow determination of the genomic region carrying a specific mutation (e.g., a mutation located in large segmental duplications). Transformation-associated recombination (TAR) cloning allows selective, most accurate, efficient, and rapid isolation of a given genomic fragment or a full-length gene from simple and complex genomes. Moreover, this method is the only way to simultaneously isolate the same genomic region from multiple individuals. As such, TAR technology is currently in a leading position to create a library of the individual genes that comprise the human genome and physically characterize the sites of chromosomal alterations (copy number variations CNVs, inversions, translocations) in the human population, associated with the predisposition to different diseases, including cancer. It is our belief that such a library and analysis of the human genome will be of great importance to the growing field of gene therapy, new drug design methods, and genomic research. In this review, we detail the motivation for TAR cloning for human genome studies, biotechnology, and biomedicine, discuss the recent progress of some TAR-based projects, and describe how TAR technology in combination with HAC (human artificial chromosome)-based and CRISPR-based technologies may contribute in the future.
An atom‐ and step‐economical and redox‐neutral cascade reaction enabled by asymmetric bimetallic relay catalysis by merging a ruthenium‐catalyzed asymmetric borrowing‐hydrogen reaction with ...copper‐catalyzed asymmetric Michael addition has been realized. A variety of highly functionalized 2‐amino‐5‐hydroxyvaleric acid esters or peptides bearing 1,4‐non‐adjacent stereogenic centers have been prepared in high yields with excellent enantio‐ and diastereoselectivity. Judicious selection and rational modification of the Ru catalysts with careful tuning of the reaction conditions played a pivotal role in stereoselectivity control as well as attenuating undesired α‐epimerization, thus enabling a full complement of all four stereoisomers that were otherwise inaccessible in previous work. Concise asymmetric stereodivergent synthesis of the key intermediates for biologically important chiral molecules further showcases the synthetic utility of this methodology.
An atom‐ and step‐economical and redox‐neutral cascade reaction enabled by dual‐metal relay catalysis by merging borrowing‐hydrogen and Michael addition reactions provided access to all stereoisomers of 2‐amino‐5‐hydroxyvaleric acid derivatives with 1,4‐non‐adjacent stereocenters. Concise stereodivergent synthesis of key intermediates for the synthesis of biologically important chiral molecules further showcases the synthetic utility of the approach.
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