AtFKBP53 is a histone chaperone required for repression of ribosomal RNA gene expression in Arabidopsis

Chromatin structure is important for controlling gene expression, but mechanisms underlying chromatin remodel- ing are not fully understood. Here we report that an FKBP （FK506 binding protein） type immunophilin, AtFKBP53, possesses histone chaperone activity and is required for repressing ribosomal gene expression in Arabidopsis. The At- FKBP53 protein is a multidomain FKBP with a typical peptidylprolyl isomerase （PPIase） domain and several highly charged domains. Using nucleosome assembly assays, we showed that AtFKBP53 has histone chaperone activity and the charged acidic domains are sufficient for the activity. We show that AtFKBP53 interacts with histone H3 through the acidic domains, whereas the PPIase domain is dispensable for histone chaperone activity or histone binding. Ri- bosomal RNA gene （18S rDNA） is overexpressed when AtFKBP53 activity is reduced or eliminated in Arabidopsis plants. Chromatin immunoprecipitation assay showed that AtFKBP53 is associated with the 18S rDNA gene chro- matin, implicating that AtFKBP53 represses rRNA genes at the chromatin level. This study identifies a new histone chaperone in plants that functions in chromatin remodeling and regulation of transcription.

RebL1 is required for macronuclear structure stability and gametogenesis in Tetrahymena thermophila

Histone modification and nucleosome assembly play important roles in chromatin-related processes.Histone chaperones form different complexes and coordinate histone transportation and assembly.Various histone chaperone complexes have been identified in different organisms.The ciliate protozoa(ciliates)have various chromatin structures and different nuclear morphology.However,histone chaperone components and functions of different subunits remain unclear in ciliates.Tet-rahymema thermophila contains a transcriptionally active macronucleus(MAC)and a transcriptionally inactive micronu-cleus(MIC)which exhibit multiple replication and various chromatin remodeling progresses during vegetative growth and sexual developmental stages.Here,we found histone chaperone RebL1 not only localized evenly in the transcriptionally active MAC but also dynamically changed in the MIC during vegetative growth and sexual developmental stages.REBL1 knockdown inhibited cellular proliferation.The macronuclear morphology became bigger in growing mutants.The abnormal macronuclear structure also occurred in the starvation stage.Furthermore,micronuclear meiosis was disturbed during sexual development,leading to a failure to generate new gametic nuclei.RebL1 potentially interacted with various factors involved in histone-modifying complexes and chromatin remodeling complexes in different developmental stages.REBL1 knockdown affected expression levels of the genes involved in chromatin organization and transcription.Taken together,RebL1 plays a vital role in maintaining macronuclear structure stability and gametogenesis in T.thermophila.

Structural basis for histone H3 recognition by NASP in Arabidopsis

The structural basis for histone recognition by the histone chaperone nuclear autoantigenic sperm protein(NASP)remains largely unclear.Here,we showed that Arabidopsis thaliana AtNASP is a monomer and displays robust nucleosome assembly activity in vitro.Examining the structure of AtNASP complexed with a histone H3α3 peptide revealed a binding mode that is conserved in human NASP.AtNASP recognizes the H3 N-terminal region distinct from human NASP.Moreover,AtNASP forms a co-chaperone complex with ANTI-SILENCING FUNCTION 1 ASF1 by binding to the H3 Nterminal region.Therefore,we deciphered the structure of AtNASP and the basis of the AtNASP-H3 interaction.

Histone Variant H3.3:A versatile H3 variant in health and in disease

Histones are the main protein components of eukaryotic chromatin. Histone variants and histone modifications modulate chromatin structure, ensuring the precise operation of cellular processes associated with genomic DNA. H3.3, an ancient and conserved H3 variant, differs from its canonical H3 counterpart by only five amino acids, yet it plays essential and specific roles in gene transcription, DNA repair and in maintaining genome integrity. Here, we review the most recent insights into the functions of histone H3.3, and the involvement of its mutant forms in human diseases.

Chromatin Remodeling in Stem Cell Maintenance in Arabidopsis thaliana

Pluripotent stem cells are able to both self-renew and generate undifferentiated cells for the formation of new tissues and organs. In higher plants, stem cells found in the shoot apical meristem （SAM） and the root apical meristem （RAM） are origins of organogenesis occurring post-embryonically. It is important to understand how the regulation of stem cell fate is coordinated to enable the meristem to constantly generate different types of lateral organs. Much knowledge has accumulated on specific transcription factors controlling SAM and RAM activity. Here, we review recent evidences for a role of chromatin remodeling in the maintenance of stable expression states of transcription factor genes and the control of stem cell activity in Arabidopsis.

Roles of chromatin assembly factor 1 in the epigenetic control of chromatin plasticity

Genetic information embedded in DNA sequence and the epigenetic information marked by modifications on DNA and his- tones are essential for the life of eukaryotes. Cells have evolved mechanisms of DNA duplication and chromatin restoration to ensure the inheritance of genetic and epigenetic information during cell division and development. In this review, we focus on the maintenance of epigenetic landscape during chromatin dynamics which requires the orchestration of histories and their chaperones. We discuss how epigenetic marks are re-established in the assembly of new chromatin after DNA replication and repair, highlighting the roles of CAF-1 in the process of changing chromatin state. The functions of CAF-1 provide a link be- tween chromatin assembly and epigenetic restoration.

A Truncated Arabidopsis NUCLEOSOME ASSEMBLY PROTEIN 1, AtNAP1;3T, Alters Plant Growth Responses to Abscisic Acid and Salt in the Atnap 1;3-2 Mutant

Chromatin remodeling is thought to have crucial roles in plant adaptive response to environmental stimulus. Here, we report that, in Arabidopsis, the evolutionarily conserved histone chaperone, NUCLEOSOME ASSEMBLY PROTEIN 1 （NAP1）, is involved in plant response to abscisic acid （ABA）, a phytohormone important in stress adaptation. We show that simultaneous loss-of-function of AtNAP1;1, AtNAP1;20 and AtNAP1;3 （the triple mutant m123-1） caused a slight hypersensitive response to ABA in seedling growth. Strikingly, the other triple mutant m123-2 containing a different mutant allele of AtNAP1;3, the Atnap 1;3-2 allele, showed a hyposensitive response to ABA and a decreased tolerance to salt stress. This ABA- hyposensitive and salt response phenotype specifically associated with the Atnapl;3-2 mutant allele. We show that this mutant allele produced a truncated protein, AtNAP1;3T, which lacks 34 amino acids at the C-terminus compared to the wild-type protein AtNAP1;3. We further show that the heterozygous plants containing the Atnapl;3-2 mutant allele as well as transgenic plants overexpressing AtNAP1;3Texhibit ABA-hyposensitive phenotype. It thus indicates that AtNAP1;3T functions as a dominant negative factor in ABA response. The expression of some ABA-responsive genes, including genes encoding protein kinases and transcription regulators, was found perturbed in the mutant and in theAtNAP1;3Ttransgenic plants. Taken together, our study uncovered AtNAP1 proteins as positive regulators and AtNAP1;3Tas a negative regulator in ABA signaling pathways, providing a novel link of chromatin remodeling to hormonal and stress responses.