The midblastula transition defines the onset of Y RNA-dependent DNA replication in Xenopus laevis

Functional long non-coding and circular RNAs in zebrafish

The utility of model organisms to understand the function of a novel transcript/genes has allowed us to delineate their molecular mechanisms in maintaining cellular homeostasis. Organisms such as zebrafish have contributed a lot in the field of developmental and disease biology. Attributable to advancement and deep transcriptomics, many new transcript isoforms and non-coding RNAs such as long noncoding RNA (lncRNA) and circular RNAs (circRNAs) have been identified and cataloged in multiple databases and many more are yet to be identified. Various methods and tools have been utilized to identify lncRNAs/circRNAs in zebrafish using deep sequencing of transcriptomes as templates. Functional analysis of a few candidates such as tie1-AS, ECAL1 and CDR1as in zebrafish provides a prospective outline to approach other known or novel lncRNA/circRNA. New genetic alteration tools like TALENS and CRISPRs have helped in probing for the molecular function of lncRNA/circRNA in zebrafish. Further latest improvements in experimental and computational techniques offer the identification of lncRNA/circRNA counterparts in humans and zebrafish thereby allowing easy modeling and analysis of function at cellular level.

Noncoding Y RNAs regulate the levels, subcellular distribution and protein interactions of their Ro60 autoantigen partner

Noncoding Y RNAs are abundant in animal cells and present in many bacteria. These RNAs are bound and stabilized by Ro60, a ring-shaped protein that is a target of autoantibodies in patients with systemic lupus erythematosus. Studies in bacteria revealed that Y RNA tethers Ro60 to a ring-shaped exoribonuclease, forming a double-ringed RNP machine specialized for structured RNA degradation. In addition to functioning as a tether, the bacterial RNA gates access of substrates to the Ro60 cavity. To identify roles for Y RNAs in mammals, we used CRISPR to generate mouse embryonic stem cells lacking one or both of the two murine Y RNAs. Despite reports that animal cell Y RNAs are essential for DNA replication, cells lacking these RNAs divide normally. However, Ro60 levels are reduced, revealing that Y RNA binding is required for Ro60 to accumulate to wild-type levels. Y RNAs regulate the subcellular location of Ro60, since Ro60 is reduced in the cytoplasm and increased in nucleoli when Y RNAs are absent. Last, we show that Y RNAs tether Ro60 to diverse effector proteins to generate specialized RNPs. Together, our data demonstrate that the roles of Y RNAs are intimately connected to that of their Ro60 partner.

Versatile interactions and bioinformatics analysis of noncoding RNAs

Advances in RNA sequencing technologies and computational methodologies have provided a huge impetus to noncoding RNA (ncRNA) study. Once regarded as inconsequential results of transcriptional promiscuity, ncRNAs were later found to exert great roles in various aspects of biological functions. They are emerging as key players in gene regulatory networks by interacting with other biomolecules (DNA, RNA or protein). Here, we provide an overview of ncRNA repertoire and highlight recent discoveries of their versatile interactions. To better investigate the ncRNA-mediated regulation, it is necessary to make full use of innovative sequencing techniques and computational tools. We further describe a comprehensive workflow for in silico ncRNA analysis, providing up-to-date platforms, databases and tools dedicated to ncRNA identification and functional annotation.

Y RNA: An Overview of Their Role as Potential Biomarkers and Molecular Targets in Human Cancers

Y RNA are a class of small non-coding RNA that are largely conserved. Although their discovery was almost 40 years ago, their function is still under investigation. This is evident in cancer biology, where their role was first studied just a dozen years ago. Since then, only a few contributions were published, mostly scattered across different tumor types and, in some cases, also suffering from methodological limitations. Nonetheless, these sparse data may be used to make some estimations and suggest routes to better understand the role of Y RNA in cancer formation and characterization. Here we summarize the current knowledge about Y RNA in multiple types of cancer, also including a paragraph about tumors that might be included in this list in the future, if more evidence becomes available. The picture arising indicates that Y RNA might be useful in tumor characterization, also relying on non-invasive methods, such as the analysis of the content of extracellular vesicles (EV) that are retrieved from blood plasma and other bodily fluids. Due to the established role of Y RNA in DNA replication, it is possible to hypothesize their therapeutic targeting to inhibit cell proliferation in oncological patients.

Genome wide decrease of DNA replication eye density at the midblastula transition of Xenopus laevis

During the first rapid divisions of early development in many species, the DNA:cytoplasm ratio increases until the midblastula transition (MBT) when transcription resumes and cell cycles lengthen. S phase is very rapid in early embryos, about 20-30 times faster than in differentiated cells. Using a combination of DNA fiber studies and a Xenopus laevis embryonic in vitro replication system, we show that S phase slows down shortly after the MBT owing to a genome wide decrease of replication eye density. Increasing the dNTP pool did not accelerate S phase or increase replication eye density implying that dNTPs are not rate limiting for DNA replication at the Xenopus MBT. Increasing the ratio of DNA:cytoplasm in egg extracts faithfully recapitulates changes in the spatial replication program in embryos, supporting the hypothesis that titration of soluble limiting factors could explain the observed changes in the DNA replication program at the MBT in Xenopus laevis.

Preserving Genome Integrity During the Early Embryonic DNA Replication Cycles

During the very early stages of embryonic development chromosome replication occurs under rather challenging conditions, including a very short cell cycle, absence of transcription, a relaxed DNA damage response and, in certain animal species, a highly contracted S-phase. This raises the puzzling question of how the genome can be faithfully replicated in such a peculiar metabolic context. Recent studies have provided new insights into this issue, and unveiled that embryos are prone to accumulate genetic and genomic alterations, most likely due to restricted cellular functions, in particular reduced DNA synthesis quality control. These findings may explain the low rate of successful development in mammals and the occurrence of diseases, such as abnormal developmental features and cancer. In this review, we will discuss recent findings in this field and put forward perspectives to further study this fascinating question.

Ro60 and Y RNAs: structure, functions, and roles in autoimmunity

Ro60, also known as SS-A or TROVE2, is an evolutionarily conserved RNA-binding protein that is found in most animal cells, approximately 5% of sequenced prokaryotic genomes and some archaea. Ro60 is present in cells as both a free protein and as a component of a ribonucleoprotein complex, where its best-known partners are members of a class of noncoding RNAs called Y RNAs. Structural and biochemical analyses have revealed that Ro60 is a ring-shaped protein that binds Y RNAs on its outer surface. In addition to Y RNAs, Ro60 binds misfolded and aberrant noncoding RNAs in some animal cell nuclei. Although the fate of these defective Ro60-bound noncoding RNAs in animal cells is not well-defined, a bacterial Ro60 ortholog functions with 3' to 5' exoribonucleases to assist structured RNA degradation. Studies of Y RNAs have revealed that these RNAs regulate the subcellular localization of Ro60, tether Ro60 to effector proteins and regulate the access of other RNAs to its central cavity. As both mammalian cells and bacteria lacking Ro60 are sensitized to ultraviolet irradiation, Ro60 function may be important during exposure to some environmental stressors. Here we summarize the current knowledge regarding the functions of Ro60 and Y RNAs in animal cells and bacteria. Because the Ro60 RNP is a clinically important target of autoantibodies in patients with rheumatic diseases such as Sjogren's syndrome, systemic lupus erythematosus, and neonatal lupus, we also discuss potential roles for Ro60 RNPs in the initiation and pathogenesis of systemic autoimmune rheumatic disease.

Identification and characterization of stem-bulge RNAs in Drosophila melanogaster

Non-coding Y RNAs and stem-bulge RNAs are homologous small RNAs in vertebrates and nematodes, respectively. They share a conserved function in the replication of chromosomal DNA in these two groups of organisms. However, functional homologues have not been found in insects, despite their common early evolutionary history. Here, we describe the identification and functional characterization of two sbRNAs in Drosophila melanogaster, termed Dm1 and Dm2. The genes coding for these two RNAs were identified by a computational search in the genome of D. melanogaster for conserved sequence motifs present in nematode sbRNAs. The predicted secondary structures of Dm1 and Dm2 partially resemble nematode sbRNAs and show stability in molecular dynamics simulations. Both RNAs are phylogenetically closer related to nematode sbRNAs than to vertebrate Y RNAs. Dm1, but not Dm2 sbRNA is abundantly expressed in D. melanogaster S2 cells and adult flies. Only Dm1, but not Dm2 sbRNA can functionally replace Y RNAs in a human cell-free DNA replication initiation system. Therefore, Dm1 is the first functional sbRNA described in insects, allowing future investigations into the physiological roles of sbRNAs in the genetically tractable model organism D. melanogaster.

On the Interplay of the DNA Replication Program and the Intra-S Phase Checkpoint Pathway

DNA replication in eukaryotes is achieved by the activation of multiple replication origins which needs to be precisely coordinated in space and time. This spatio-temporal replication program is regulated by many factors to maintain genome stability, which is frequently threatened through stresses of exogenous or endogenous origin. Intra-S phase checkpoints monitor the integrity of DNA synthesis and are activated when replication forks are stalled. Their activation leads to the stabilization of forks, to the delay of the replication program by the inhibition of late firing origins, and the delay of G2/M phase entry. In some cell cycles during early development these mechanisms are less efficient in order to allow rapid cell divisions. In this article, we will review our current knowledge of how the intra-S phase checkpoint regulates the replication program in budding yeast and metazoan models, including early embryos with rapid S phases. We sum up current models on how the checkpoint can inhibit origin firing in some genomic regions, but allow dormant origin activation in other regions. Finally, we discuss how numerical and theoretical models can be used to connect the multiple different actors into a global process and to extract general rules.

Bacterial Y RNAs: Gates, Tethers, and tRNA Mimics

Y RNAs are noncoding RNAs (ncRNAs) that are present in most animal cells and also in many bacteria. These RNAs were discovered because they are bound by the Ro60 protein, a major target of autoantibodies in patients with some systemic autoimmune rheumatic diseases. Studies of Ro60 and Y RNAs in Deinococcus radiodurans, the first sequenced bacterium with a Ro60 ortholog, revealed that they function with 3'-to-5' exoribonucleases to alter the composition of RNA populations during some forms of environmental stress. In the best-characterized example, Y RNA tethers the Ro60 protein to the exoribonuclease polynucleotide phosphorylase, allowing this exoribonuclease to degrade structured RNAs more effectively. Y RNAs can also function as gates to regulate access of other RNAs to the Ro60 central cavity. Recent studies in the enteric bacterium Salmonella enterica serovar Typhimurium resulted in the discovery that Y RNAs are widely present in bacteria. Remarkably, the most-conserved subclass of bacterial Y RNAs contains a domain that mimics tRNA. In this review, we discuss the structure, conservation, and known functions of bacterial Y RNAs as well as the certainty that more bacterial Y RNAs and additional roles for these ncRNAs remain to be uncovered.

Platelet-Enriched MicroRNAs and Cardiovascular Homeostasis

SIGNIFICANCE

Platelets are anucleate blood cells that are involved in hemostasis and thrombosis. Although no longer able to generate ribonucleic acid (RNA) de novo, platelets contain messenger RNA (mRNA), YRNA fragments, and premature microRNAs (miRNAs) that they inherit from megakaryocytes. Recent Advances: Novel sequencing techniques have helped identify the unexpectedly large number of RNA species present in platelets. Throughout their life time, platelets can process the pre-existing pool of premature miRNA to give the fully functional miRNA that can regulate platelet protein expression and function.

CRITICAL ISSUES

Platelets make a major contribution to the circulating miRNA pool but platelet activation can have major consequences on Dicer levels and thus miRNA maturation, which has implications for studies that are focused on screening-stored platelets.

FUTURE DIRECTIONS

It will be important to determine the importance of platelets as donors for miRNA-containing microvesicles that can be taken up and processed by other (particularly vascular) cells, thus contributing to homeostasis as well as disease progression. Antioxid. Redox Signal. 29, 902-921.

Innate Immune Response and Off-Target Mis-splicing Are Common Morpholino-Induced Side Effects in Xenopus

Antisense morpholino oligomers (MOs) have been indispensable tools for developmental biologists to transiently knock down (KD) genes rather than to knock them out (KO). Here we report on the implications of genetic KO versus MO-mediated KD of the mesoderm-specifying Brachyury paralogs in the frog Xenopus tropicalis. While both KO and KD embryos fail to activate the same core gene regulatory network, resulting in virtually identical morphological defects, embryos injected with control or target MOs also show a systemic GC content-dependent immune response and many off-target splicing defects. Optimization of MO dosage and increasing incubation temperatures can mitigate, but not eliminate, these MO side effects, which are consistent with the high affinity measured between MO and off-target sequence in vitro. We conclude that while MOs can be useful to profile loss-of-function phenotypes at a molecular level, careful attention must be paid to their immunogenic and off-target side effects.

A NuRD Complex from Xenopus laevis Eggs Is Essential for DNA Replication during Early Embryogenesis

DNA replication in the embryo of Xenopus laevis changes dramatically at the mid-blastula transition (MBT), with Y RNA-independent random initiation switching to Y RNA-dependent initiation at specific origins. Here, we identify xNuRD, an MTA2-containing assemblage of the nucleosome remodeling and histone deacetylation complex NuRD, as an essential factor in pre-MBT Xenopus embryos that overcomes a functional requirement for Y RNAs during DNA replication. Human NuRD complexes have a different subunit composition than xNuRD and do not support Y RNA-independent initiation of DNA replication. Blocking or immunodepletion of xNuRD inhibits DNA replication initiation in isolated nuclei in vitro and causes inhibition of DNA synthesis, developmental delay, and embryonic lethality in early embryos. xNuRD activity declines after the MBT, coinciding with dissociation of the complex and emergence of Y RNA-dependent initiation. Our data thus reveal an essential role for a NuRD complex as a DNA replication factor during early Xenopus development.

Targeting Functional Noncoding RNAs

Noncoding RNAs have essential biochemical functions in different areas of cellular metabolism, including protein synthesis, RNA splicing, protein secretion, and DNA replication. We have successfully used Morpholino antisense oligonucleotides for the functional inactivation of small noncoding RNAs required for DNA replication (Y RNAs in vertebrates and stem-bulge RNAs in nematodes). Here we discuss specific issues of targeting functional noncoding RNAs for inactivation by Morpholino antisense oligonucleotides. We present protocols for the design, preparation, and efficacy controls of Morpholino antisense oligonucleotides, as well as brief descriptions for their delivery into vertebrate and nematode embryos.

RNA secondary structure profiling in zebrafish reveals unique regulatory features

Background

RNA is known to play diverse roles in gene regulation. The clues for this regulatory function of RNA are embedded in its ability to fold into intricate secondary and tertiary structure.

Results

We report the transcriptome-wide RNA secondary structure in zebrafish at single nucleotide resolution using Parallel Analysis of RNA Structure (PARS). This study provides the secondary structure map of zebrafish coding and non-coding RNAs. The single nucleotide pairing probabilities of 54,083 distinct transcripts in the zebrafish genome were documented. We identified RNA secondary structural features embedded in functional units of zebrafish mRNAs. Translation start and stop sites were demarcated by weak structural signals. The coding regions were characterized by the three-nucleotide periodicity of secondary structure and display a codon base specific structural constrain. The splice sites of transcripts were also delineated by distinct signature signals. Relatively higher structural signals were observed at 3’ Untranslated Regions (UTRs) compared to Coding DNA Sequence (CDS) and 5’ UTRs. The 3′ ends of transcripts were also marked by unique structure signals. Secondary structural signals in long non-coding RNAs were also explored to better understand their molecular function.

Conclusions

Our study presents the first PARS-enabled transcriptome-wide secondary structure map of zebrafish, which documents pairing probability of RNA at single nucleotide precision. Our findings open avenues for exploring structural features in zebrafish RNAs and their influence on gene expression.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4497-0) contains supplementary material, which is available to authorized users.

Nonreplicative functions of the origin recognition complex

Origin recognition complex (ORC), a heteromeric six-subunit complex, is the central component of the eukaryotic pre-replication complex. Recent data from yeast, frogs, flies and mammals present compelling evidence that ORC and its individual subunits have nonreplicative functions as well. The majority of these functions, such as heterochromatin formation, chromosome condensation, and segregation are dependent on ORC-DNA interactions. Furthermore, ORC is involved in the control of cell division via its participation in centrosome duplication and cytokinesis. Recent findings have also demonstrated a direct interaction between ORC and mRNPs and highlighted an essential role of ORC in mRNA nuclear export. Along with the growth of evolutionary complexity of organisms, ORC complex functions become more elaborate and new functions of the ORC sub-complexes and individual subunits have emerged.

Non-coding Y RNAs associate with early replicating euchromatin in concordance with the origin recognition complex

Non-coding Y RNAs are essential for the initiation of chromosomal DNA replication in vertebrates, yet their association with chromatin during the cell cycle is not characterised. Here, we quantify human Y RNA levels in soluble and chromatin-associated intracellular fractions and investigate, topographically, their dynamic association with chromatin during the cell cycle. We find that, on average, about a million Y RNA molecules are present in the soluble fraction of a proliferating cell, and 5-10-fold less are in association with chromatin. These levels decrease substantially during quiescence. No significant differences are apparent between cancer and non-cancer cell lines. Y RNAs associate with euchromatin throughout the cell cycle. Their levels are 2-4-fold higher in S phase than in G1 phase or mitosis. Y RNAs are not detectable at active DNA replication foci, and re-associate with replicated euchromatin during mid and late S phase. The dynamics and sites of Y1 RNA association with chromatin are in concordance with those of the origin recognition complex (ORC). Our data therefore suggest a functional role of Y RNAs in a common pathway with ORC.

ORC interacts with THSC/TREX-2 and its subunits promote Nxf1 association with mRNP and mRNA export in Drosophila

The origin recognition complex (ORC) of eukaryotes associates with the replication origins and initiates the pre-replication complex assembly. In the literature, there are several reports of interaction of ORC with different RNAs. Here, we demonstrate for the first time a direct interaction of ORC with the THSC/TREX-2 mRNA nuclear export complex. The THSC/TREX-2 was purified from the Drosophila embryonic extract and found to bind with a fraction of the ORC. This interaction occurred via several subunits and was essential for Drosophila viability. Also, ORC was associated with mRNP, which was facilitated by TREX-2. ORC subunits interacted with the Nxf1 receptor mediating the bulk mRNA export. The knockdown of Orc5 led to a drop in the Nxf1 association with mRNP, while Orc3 knockdown increased the level of mRNP-bound Nxf1. The knockdown of Orc5, Orc3 and several other ORC subunits led to an accumulation of mRNA in the nucleus, suggesting that ORC participates in the regulation of the mRNP export.

Structural and functional analysis of four non-coding Y RNAs from Chinese hamster cells: identification, molecular dynamics simulations and DNA replication initiation assays

BACKGROUND

The genes coding for Y RNAs are evolutionarily conserved in vertebrates. These non-coding RNAs are essential for the initiation of chromosomal DNA replication in vertebrate cells. However thus far, no information is available about Y RNAs in Chinese hamster cells, which have already been used to detect replication origins and alternative DNA structures around these sites. Here, we report the gene sequences and predicted structural characteristics of the Chinese hamster Y RNAs, and analyze their ability to support the initiation of chromosomal DNA replication in vitro.

RESULTS

We identified DNA sequences in the Chinese hamster genome of four Y RNAs (chY1, chY3, chY4 and chY5) with upstream promoter sequences, which are homologous to the four main types of vertebrate Y RNAs. The chY1, chY3 and chY5 genes were highly conserved with their vertebrate counterparts, whilst the chY4 gene showed a relatively high degree of diversification from the other vertebrate Y4 genes. Molecular dynamics simulations suggest that chY4 RNA is structurally stable despite its evolutionarily divergent predicted stem structure. Of the four Y RNA genes present in the hamster genome, we found that only the chY1 and chY3 RNA were strongly expressed in the Chinese hamster GMA32 cell line, while expression of the chY4 and chY5 RNA genes was five orders of magnitude lower, suggesting that they may in fact not be expressed. We synthesized all four chY RNAs and showed that any of these four could support the initiation of DNA replication in an established human cell-free system.

CONCLUSIONS

These data therefore establish that non-coding chY RNAs are stable structures and can substitute for human Y RNAs in a reconstituted cell-free DNA replication initiation system. The pattern of Y RNA expression and functionality is consistent with Y RNAs of other rodents, including mouse and rat.

DNA replication origin activation in space and time

DNA replication begins with the assembly of pre-replication complexes (pre-RCs) at thousands of DNA replication origins during the G1 phase of the cell cycle. At the G1-S-phase transition, pre-RCs are converted into pre-initiation complexes, in which the replicative helicase is activated, leading to DNA unwinding and initiation of DNA synthesis. However, only a subset of origins are activated during any S phase. Recent insights into the mechanisms underlying this choice reveal how flexibility in origin usage and temporal activation are linked to chromosome structure and organization, cell growth and differentiation, and replication stress.

Short stories on zebrafish long noncoding RNAs

The recent re-annotation of the transcriptome of human and other model organisms, using next-generation sequencing approaches, has unravelled a hitherto unknown repertoire of transcripts that do not have a potential to code for proteins. These transcripts have been largely classified into an amorphous class popularly known as long noncoding RNAs (lncRNA). This discovery of lncRNAs in human and other model systems have added a new layer to the understanding of gene regulation at the transcriptional and post-transcriptional levels. In recent years, three independent studies have discovered a number of lncRNAs expressed in different stages of zebrafish development and adult tissues using a high-throughput RNA sequencing approach, significantly adding to the repertoire of genes known in zebrafish. A subset of these transcripts also shows distinct and specific spatiotemporal patterns of gene expression, pointing to a tight regulatory control and potential functional roles in development, organogenesis, and/ or homeostasis. This review provides an overview of the lncRNAs in zebrafish and discusses how their discovery could provide new insights into understanding biology, explaining mutant phenotypes, and helping in potentially modeling disease processes.

Functional roles of non-coding Y RNAs

Non-coding RNAs are involved in a multitude of cellular processes but the biochemical function of many small non-coding RNAs remains unclear. The family of small non-coding Y RNAs is conserved in vertebrates and related RNAs are present in some prokaryotic species. Y RNAs are also homologous to the newly identified family of non-coding stem-bulge RNAs (sbRNAs) in nematodes, for which potential physiological functions are only now emerging. Y RNAs are essential for the initiation of chromosomal DNA replication in vertebrates and, when bound to the Ro60 protein, they are involved in RNA stability and cellular responses to stress in several eukaryotic and prokaryotic species. Additionally, short fragments of Y RNAs have recently been identified as abundant components in the blood and tissues of humans and other mammals, with potential diagnostic value. While the number of functional roles of Y RNAs is growing, it is becoming increasingly clear that the conserved structural domains of Y RNAs are essential for distinct cellular functions. Here, we review the biochemical functions associated with these structural RNA domains, as well as the functional conservation of Y RNAs in different species. The existing biochemical and structural evidence supports a domain model for these small non-coding RNAs that has direct implications for the modular evolution of functional non-coding RNAs.

Non-coding stem-bulge RNAs are required for cell proliferation and embryonic development in C. elegans

Stem bulge RNAs (sbRNAs) are a family of small non-coding stem-loop RNAs present in Caenorhabditis elegans and other nematodes, the function of which is unknown. Here, we report the first functional characterisation of nematode sbRNAs. We demonstrate that sbRNAs from a range of nematode species are able to reconstitute the initiation of chromosomal DNA replication in the presence of replication proteins in vitro, and that conserved nucleotide sequence motifs are essential for this function. By functionally inactivating sbRNAs with antisense morpholino oligonucleotides, we show that sbRNAs are required for S phase progression, early embryonic development and the viability of C. elegans in vivo. Thus, we demonstrate a new and essential role for sbRNAs during the early development of C. elegans. sbRNAs show limited nucleotide sequence similarity to vertebrate Y RNAs, which are also essential for the initiation of DNA replication. Our results therefore establish that the essential function of small non-coding stem-loop RNAs during DNA replication extends beyond vertebrates.

New insights into the maternal to zygotic transition

The initial phases of embryonic development occur in the absence of de novo transcription and are instead controlled by maternally inherited mRNAs and proteins. During this initial period, cell cycles are synchronous and lack gap phases. Following this period of transcriptional silence, zygotic transcription begins, the maternal influence on development starts to decrease, and dramatic changes to the cell cycle take place. Here, we discuss recent work that is shedding light on the maternal to zygotic transition and the interrelated but distinct mechanisms regulating the onset of zygotic transcription and changes to the cell cycle during early embryonic development.

Nucleotide contributions to the structural integrity and DNA replication initiation activity of noncoding y RNA

Noncoding Y RNAs are small stem-loop RNAs that are involved in different cellular processes, including the regulation of DNA replication. An evolutionarily conserved small domain in the upper stem of vertebrate Y RNAs has an essential function for the initiation of chromosomal DNA replication. Here we provide a structure-function analysis of this essential RNA domain under physiological conditions. Solution state nuclear magnetic resonance and far-ultraviolet circular dichroism spectroscopy show that the upper stem domain of human Y1 RNA adopts a locally destabilized A-form helical structure involving eight Watson-Crick base pairs. Within this helix, two G:C base pairs are highly stable even at elevated temperatures and therefore may serve as clamps to maintain the local structure of the helix. These two stable G:C base pairs frame three unstable base pairs, which are located centrally between them. Systematic substitution mutagenesis results in a disruption of the ordered A-form helical structure and in the loss of DNA replication initiation activity, establishing a positive correlation between folding stability and function. Our data thus provide a structural basis for the evolutionary conservation of key nucleotides in this RNA domain that are essential for the functionality of noncoding Y RNAs during the initiation of DNA replication.

Circulating small noncoding RNAs as biomarkers of aging

Small noncoding RNAs (sncRNAs) mediate a variety of cellular functions in animals and plants. Deep sequencing has made it possible to obtain highly detailed information on the types and abundance of sncRNAs in biological specimens, leading to the discovery that sncRNAs circulate in the blood of humans and mammals. The most abundant types of circulating sncRNAs are microRNAs (miRNAs), 5' transfer RNA (tRNA) halves, and YRNA fragments, with minute amounts of other types that may nevertheless be significant. Of the more abundant circulating sncRNAs only miRNAs have well described functions, but characteristics of the others suggest specific processing and secretion as complexes that protect the RNA from degradation. The properties of circulating sncRNAs are consistent with their serving as signaling molecules, and investigations of circulating miRNAs support the view that they can enter cells and regulate cellular functions. The serum levels of specific sncRNAs change markedly with age, and these changes can be mitigated by calorie restriction (CR), indicating that levels are under physiologic control. The ability of circulating sncRNAs to transmit functions between cells and to regulate a broad spectrum of cellular functions, and the changes in their levels with age, implicate them in the manifestations of aging. Our understanding of the functions of circulating sncRNA, particularly in relation to aging, is currently at a very early stage; results to date suggest that more extensive investigation will yield important insights into mechanisms of aging.

Noncoding RNAs in the regulation of DNA replication

Noncoding RNAs (ncRNAs) have crucial roles in epigenetic, transcriptional, and post-transcriptional regulation. Recent studies have begun to reveal a role of ncRNAs in DNA replication. Here, we review the roles of ncRNAs in regulating different aspects of DNA replication in prokaryotic and eukaryotic systems. We speculate that ncRNAs might function to guide the origin recognition complex (ORC) to chromosomal DNA during replication initiation in higher eukaryotes.

Non-coding Y RNAs as tethers and gates: Insights from bacteria

Non-coding RNAs (ncRNAs) called Y RNAs are abundant components of both animal cells and a variety of bacteria. In all species examined, these ~100 nt RNAs are bound to the Ro 60 kDa (Ro60) autoantigen, a ring-shaped protein that also binds misfolded ncRNAs in some vertebrate nuclei. Although the function of Ro60 RNPs has been mysterious, we recently reported that a bacterial Y RNA tethers Ro60 to the 3' to 5' exoribonuclease polynucleotide phosphorylase (PNPase) to form RYPER (Ro60/Y RNA/PNPase Exoribonuclease RNP), a new RNA degradation machine. PNPase is a homotrimeric ring that degrades single-stranded RNA, and Y RNA-mediated tethering of Ro60 increases the effectiveness of PNPase in degrading structured RNAs. Single particle electron microscopy of RYPER suggests that RNA threads through the Ro60 ring into the PNPase cavity. Further studies indicate that Y RNAs may also act as gates to regulate entry of RNA substrates into the Ro60 channel. These findings reveal novel functions for Y RNAs and raise questions about how the bacterial findings relate to the roles of these ncRNAs in animal cells. Here we review the literature on Y RNAs, highlighting their close relationship with Ro60 proteins and the hypothesis that these ncRNAs function generally to tether Ro60 rings to diverse RNA-binding proteins.

Why YRNAs? About Versatile RNAs and Their Functions

Y RNAs constitute a family of highly conserved small noncoding RNAs (in humans: 83-112 nt; Y1, Y3, Y4 and Y5). They are transcribed from individual genes by RNA-polymerase III and fold into conserved stem-loop-structures. Although discovered 30 years ago, insights into the cellular and physiological role of Y RNAs remains incomplete. In this review, we will discuss knowledge on the structural properties, associated proteins and discuss proposed functions of Y RNAs. We suggest Y RNAs to be an integral part of ribonucleoprotein networks within cells and could therefore have substantial influence on many different cellular processes. Putative functions of Y RNAs include small RNA quality control, DNA replication, regulation of the cellular stress response and proliferation. This suggests Y RNAs as essential regulators of cell fate and indicates future avenues of research, which will provide novel insights into the role of small noncoding RNAs in gene expression.

Genome-wide mapping of human DNA-replication origins: levels of transcription at ORC1 sites regulate origin selection and replication timing

We report the genome-wide mapping of ORC1 binding sites in mammals, by chromatin immunoprecipitation and parallel sequencing (ChIP-seq). ORC1 binding sites in HeLa cells were validated as active DNA replication origins (ORIs) using Repli-seq, a method that allows identification of ORI-containing regions by parallel sequencing of temporally ordered replicating DNA. ORC1 sites were universally associated with transcription start sites (TSSs) of coding or noncoding RNAs (ncRNAs). Transcription levels at the ORC1 sites directly correlated with replication timing, suggesting the existence of two classes of ORIs: those associated with moderate/high transcription levels (≥1 RNA copy/cell), firing in early S and mapping to the TSSs of coding RNAs; and those associated with low transcription levels (<1 RNA copy/cell), firing throughout the entire S and mapping to TSSs of ncRNAs. These findings are compatible with a scenario whereby TSS expression levels influence the efficiency of ORC1 recruitment at G₁ and the probability of firing during S.

Emerging players in the initiation of eukaryotic DNA replication

Faithful duplication of the genome in eukaryotes requires ordered assembly of a multi-protein complex called the pre-replicative complex (pre-RC) prior to S phase; transition to the pre-initiation complex (pre-IC) at the beginning of DNA replication; coordinated progression of the replisome during S phase; and well-controlled regulation of replication licensing to prevent re-replication. These events are achieved by the formation of distinct protein complexes that form in a cell cycle-dependent manner. Several components of the pre-RC and pre-IC are highly conserved across all examined eukaryotic species. Many of these proteins, in addition to their bona fide roles in DNA replication are also required for other cell cycle events including heterochromatin organization, chromosome segregation and centrosome biology. As the complexity of the genome increases dramatically from yeast to human, additional proteins have been identified in higher eukaryotes that dictate replication initiation, progression and licensing. In this review, we discuss the newly discovered components and their roles in cell cycle progression.