抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

50 Years of Image Analysis

Eukaryotic genomes are packaged into nucleosome particles that occlude the DNA from interacting with most DNA binding proteins. Nucleosomes have higher affinity for particular DNA sequences, reflecting the ability of the sequence to bend sharply, as required by the nucleosome structure. However, it is not known whether these sequence preferences have a significant influence on nucleosome position in vivo, and thus regulate the access of other proteins to DNA. Here we isolated nucleosome-bound sequences at high resolution from yeast and used these sequences in a new computational approach to construct and validate experimentally a nucleosome–DNA interaction model, and to predict the genome-wide organization of nucleosomes. Our results demonstrate that genomes encode an intrinsic nucleosome organization and that this intrinsic organization can explain ∼50% of the in vivo nucleosome positions. This nucleosome positioning code may facilitate specific chromosome functions including transcription factor binding, transcription initiation, and even remodelling of the nucleosomes themselves.

http://absimage.aps.org/image/MAR08/MWS_MAR08-2008-020581.pdf

The Genomic Code for Nucleosome Positioning

Cells utilize diverse ATP-dependent nucleosome-remodelling complexes to carry out histone sliding, ejection or the incorporation of histone variants, suggesting that different mechanisms of action are used by the various chromatin-remodelling complex subfamilies. However, all chromatin-remodelling complex subfamilies contain an ATPase-translocase 'motor' that translocates DNA from a common location within the nucleosome. In this Review, we discuss (and illustrate with animations) an alternative, unifying mechanism of chromatin remodelling, which is based on the regulation of DNA translocation. We propose the 'hourglass' model of remodeller function, in which each remodeller subfamily utilizes diverse specialized proteins and protein domains to assist in nucleosome targeting or to differentially detect nucleosome epitopes. These modules converge to regulate a common DNA translocation mechanism, to inform the conserved ATPase 'motor' on whether and how to apply DNA translocation, which together achieve the various outcomes of chromatin remodelling: nucleosome assembly, chromatin access and nucleosome editing.

Mechanisms of action and regulation of ATP-dependent chromatin-remodelling complexes

The role of long noncoding RNA (lncRNA) in adult hearts is unknown; also unclear is how lncRNA modulates nucleosome remodelling. An estimated 70% of mouse genes undergo antisense transcription, including myosin heavy chain 7 (Myh7), which encodes molecular motor proteins for heart contraction. Here we identify a cluster of lncRNA transcripts from Myh7 loci and demonstrate a new lncRNA-chromatin mechanism for heart failure. In mice, these transcripts, which we named myosin heavy-chain-associated RNA transcripts (Myheart, or Mhrt), are cardiac-specific and abundant in adult hearts. Pathological stress activates the Brg1-Hdac-Parp chromatin repressor complex to inhibit Mhrt transcription in the heart. Such stress-induced Mhrt repression is essential for cardiomyopathy to develop: restoring Mhrt to the pre-stress level protects the heart from hypertrophy and failure. Mhrt antagonizes the function of Brg1, a chromatin-remodelling factor that is activated by stress to trigger aberrant gene expression and cardiac myopathy. Mhrt prevents Brg1 from recognizing its genomic DNA targets, thus inhibiting chromatin targeting and gene regulation by Brg1. It does so by binding to the helicase domain of Brg1, a domain that is crucial for tethering Brg1 to chromatinized DNA targets. Brg1 helicase has dual nucleic-acid-binding specificities: it is capable of binding lncRNA (Mhrt) and chromatinized--but not naked--DNA. This dual-binding feature of helicase enables a competitive inhibition mechanism by which Mhrt sequesters Brg1 from its genomic DNA targets to prevent chromatin remodelling. A Mhrt-Brg1 feedback circuit is thus crucial for heart function. Human MHRT also originates from MYH7 loci and is repressed in various types of myopathic hearts, suggesting a conserved lncRNA mechanism in human cardiomyopathy. Our studies identify a cardioprotective lncRNA, define a new targeting mechanism for ATP-dependent chromatin-remodelling factors, and establish a new paradigm for lncRNA-chromatin interaction.

/pdf/a-long-noncoding-rna-protects-the-heart-from-pathological-1hsjtq0nqi.pdf

A long noncoding RNA protects the heart from pathological hypertrophy

Nucleosomes are efficient DNA-packaging units. The fundamental protein unit of the nucleosome is the histone dimer, a simple α-helical domain possessing a highly basic, curved surface that closely matches the phosphate backbone of bent duplex DNA. Two copies each of histone heterodimer, H3/H4 and H2A/H2B, form a histone octamer that is wrapped with approximately 146 bp of duplex DNA in a left-handed spiral1,2 (Figure ​(Figure1).1). Through extensive electrostatic and hydrogen-bonding interactions, each histone dimer coordinates three consecutive minor grooves on the inner surface of the DNA spiral. The bending of DNA over the protein surface brings the phosphate backbone of the two strands closer together on the inside of the spiral, narrowing the major and minor grooves of DNA, while widening the grooves on the outside. This bent conformation of the DNA duplex, which would otherwise be energetically unfavorable, is maintained through charge neutralization from numerous arginine and lysine side chains of the histones.




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Figure 1


Overview of nucleosome architecture. (A) Illustration of H2A/H2B and H3/H4 heterodimers and how they fit together to form the histone octamer. (B) Face and top view of the nucleosome structure. For this and all subsequent molecular representations of the nucleosome, the high-resolution crystal structure (PDB code 1KX5) was used.93

Post-translational modifications of histones that influence nucleosome dynamics.

The chromodomains of the Chd1 chromatin remodeler regulate DNA access to the ATPase motor

Apurinic/apyrimidinic (AP) sites are ubiquitous DNA lesions that are highly mutagenic and cytotoxic if not repaired. In addition, clusters of two or more abasic lesions within one to two turns of DNA, a hallmark of ionizing radiation, are repaired much less efficiently and thus present greater mutagenic potential. Abasic sites are chemically labile, but naked DNA containing them undergoes strand scission slowly with a half-life on the order of weeks. We find that independently generated AP sites within nucleosome core particles are highly destabilized, with strand scission occurring ∼60-fold more rapidly than in naked DNA. The majority of core particles containing single AP lesions accumulate DNA–protein cross-links, which persist following strand scission. The N-terminal region of histone protein H4 contributes significantly to DNA–protein cross-links and strand scission when AP sites are produced approximately 1.5 helical turns from the nucleosome dyad, which is a known hot spot for nucleosomal DNA damage. Reaction rates for AP sites at two positions within this region differ by ∼4-fold. However, the strand scission of the slowest reacting AP site is accelerated when it is part of a repair resistant bistranded lesion composed of two AP sites, resulting in rapid formation of double strand breaks in high yields. Multiple lysine residues within a single H4 protein catalyze double strand cleavage through a mechanism believed to involve a templating effect. These results show that AP sites within the nucleosome produce significant amounts of DNA–protein cross-links and generate double strand breaks, the most deleterious form of DNA damage.

https://www.pnas.org/content/pnas/107/52/22475.full.pdf

Rapid DNA-protein cross-linking and strand scission by an abasic site in a nucleosome core particle

Mechanisms of ATP-dependent nucleosome sliding

Chd1- and ISWI-type chromatin remodelers can sense extranucleosomal DNA and preferentially shift nucleosomes toward longer stretches of available DNA. The DNA-binding domains of these chromatin remodelers are believed to be responsible for sensing extranucleosomal DNA and are needed for robust sliding, but it is unclear how these domains contribute to directional movement of nucleosomes. Here, we show that the DNA-binding domain of Chd1 is not essential for nucleosome sliding but is critical for centering mononucleosomes on short DNA fragments. Remarkably, nucleosome centering was achieved by replacing the native DNA-binding domain of Chd1 with foreign DNA-binding domains of Escherichia coli AraC or Drosophila melanogaster engrailed. Introducing target DNA sequences recognized by the foreign domains enabled the remodelers to rapidly shift nucleosomes toward these binding sites, demonstrating that these foreign DNA-binding domains dictated the direction of sliding. Sequence-directed sliding occluded the target DNA sequences on the nucleosome enough to promote release of the remodeler. Target DNA sequences were highly stimulatory at multiple positions flanking the nucleosome and had the strongest influence when separated from the nucleosome by 23 or fewer base pairs. These results suggest that the DNA-binding domain's affinity for extranucleosomal DNA is the key determinant for the direction that Chd1 shifts the nucleosome.

Gregory D. Bowman

Papers

Post-translational modifications of histones that influence nucleosome dynamics.

The chromodomains of the Chd1 chromatin remodeler regulate DNA access to the ATPase motor

Rapid DNA-protein cross-linking and strand scission by an abasic site in a nucleosome core particle

Mechanisms of ATP-dependent nucleosome sliding

Extranucleosomal DNA Binding Directs Nucleosome Sliding by Chd1