Genome-wide view of natural antisense transcripts in Arabidopsis thaliana

Genome-wide view and characterization of natural antisense transcripts in Cannabis Sativa L

Natural Antisense Transcripts (NATs) are a kind of complex regulatory RNAs that play crucial roles in gene expression and regulation. However, the NATs in Cannabis Sativa L., a widely economic and medicinal plant rich in cannabinoids remain unknown. In this study, we comprehensively predicted C. sativa NATs genome-wide using strand-specific RNA sequencing (ssRNA-Seq) data, and validated the expression profiles by strand-specific quantitative reverse transcription PCR (ssRT-qPCR). Consequently, a total of 307 NATs were predicted in C. sativa, including 104 cis- and 203 trans- NATs. Functional enrichment analysis demonstrated the potential involvement of the C. sativa NATs in DNA polymerase activity, RNA-DNA hybrid ribonuclease activity, and nucleic acid binding. Finally, 18 cis- and 376 trans- NAT-ST pairs were predicted to produce 621 cis- and 5,679 trans- small interfering RNA (nat-siRNAs), respectively. These nat-siRNAs were potentially involved in the biosynthesis of cannabinoids and cellulose. All these results will shed light on the regulation of NATs and nat-siRNAs in C. sativa.

Population-level annotation of lncRNAs in Arabidopsis reveals extensive expression variation associated with transposable element-like silencing

Long noncoding RNAs (lncRNAs) are understudied and underannotated in plants. In mammals, lncRNA loci are nearly as ubiquitous as protein-coding genes, and their expression is highly variable between individuals of the same species. Using Arabidopsis thaliana as a model, we aimed to elucidate the true scope of lncRNA transcription across plants from different regions and study its natural variation. We used transcriptome deep sequencing data sets spanning hundreds of natural accessions and several developmental stages to create a population-wide annotation of lncRNAs, revealing thousands of previously unannotated lncRNA loci. While lncRNA transcription is ubiquitous in the genome, most loci appear to be actively silenced and their expression is extremely variable between natural accessions. This high expression variability is largely caused by the high variability of repressive chromatin levels at lncRNA loci. High variability was particularly common for intergenic lncRNAs (lincRNAs), where pieces of transposable elements (TEs) present in 50% of these lincRNA loci are associated with increased silencing and variation, and such lncRNAs tend to be targeted by the TE silencing machinery. We created a population-wide lncRNA annotation in Arabidopsis and improve our understanding of plant lncRNA genome biology, raising fundamental questions about what causes transcription and silencing across the genome.

BioGraph: Data Model for Linking and Querying Diverse Biological Metadata

Studying the association of gene function, diseases, and regulatory gene network reconstruction demands data compatibility. Data from different databases follow distinct schemas and are accessible in heterogenic ways. Although the experiments differ, data may still be related to the same biological entities. Some entities may not be strictly biological, such as geolocations of habitats or paper references, but they provide a broader context for other entities. The same entities from different datasets can share similar properties, which may or may not be found within other datasets. Joint, simultaneous data fetching from multiple data sources is complicated for the end-user or, in many cases, unsupported and inefficient due to differences in data structures and ways of accessing the data. We propose BioGraph-a new model that enables connecting and retrieving information from the linked biological data that originated from diverse datasets. We have tested the model on metadata collected from five diverse public datasets and successfully constructed a knowledge graph containing more than 17 million model objects, of which 2.5 million are individual biological entity objects. The model enables the selection of complex patterns and retrieval of matched results that can be discovered only by joining the data from multiple sources.

The non-coding RNA SVALKA locus produces a cis-natural antisense transcript that negatively regulates the expression of CBF1 and biomass production at normal temperatures

Non-coding transcription is present in all eukaryotic genomes, but we lack fundamental knowledge about its importance for an organism's ability to develop properly. In plants, emerging evidence highlights the essential biological role of non-coding transcription in the regulation of coding transcription. However, we have few molecular insights into this regulation. Here, we show that a long isoform of the long non-coding RNA SVALKA-L (SVK-L) forms a natural antisense transcript to the host gene CBF1 and negatively regulates CBF1 mRNA levels at normal temperatures in the model plant Arabidopsis thaliana. Furthermore, we show detailed evidence for the specific mode of action of SVK-L. This pathway includes the formation of double-stranded RNA that is recognized by the DICER proteins and subsequent downregulation of CBF1 mRNA levels. Thus, the CBF1-SVK regulatory circuit is not only important for its previously known role in cold temperature acclimation but also for biomass production at normal temperatures. Our study characterizes the developmental role of SVK-L and offers mechanistic insight into how biologically important overlapping natural antisense transcripts can act on and fine-tune the steady-state levels of their host gene's mRNA.

Regulation of small RNA-mediated high temperature stress responses in crop plants

Small RNAs have emerged as key players of gene expression regulation. Several lines of evidences highlight their role in modulating high temperature stress responsiveness in plants. Throughout their life cycle, plants have to regulate their gene expression at various developmental phases, physiological changes, and in response to biotic or environmental stress. High temperature is one the most common abiotic stress for crop plants, that results in impaired morphology, physiology, and yield. However, plants have certain mechanisms that enable them to withstand such conditions by modulating the expression of stress-related genes. Small RNA (sRNA)-regulated gene expression is one such mechanism which is ubiquitous in all eukaryotes. The sRNAs mainly include micro RNAs (miRNAs) and small interfering RNAs (siRNAs). They are primarily associated with the gene silencing either through translation inhibition, mRNA degradation, or DNA methylation. During high temperature stress the increased or decreased level of miRNAs altered the protein accumulation of target transcripts and, therefore, regulate stress responses. Several reports are available in plants which are genetically engineered through expressing artificial miRNAs resulted in thermotolerance. sRNAs have also been reported to bring the epigenetic changes on chromatin region through RNA-dependent DNA methylation (RdDM). The present article draws a brief illustration of sRNA origin, their functional mechanisms, role in high temperature stress, and possible application for developing stress tolerant crop plants.

Plant RNA-mediated gene regulatory network

Not all transcribed RNAs are protein-coding RNAs. Many of them are non-protein-coding RNAs in diverse eukaryotes. However, some of them seem to be non-functional and are resulted from spurious transcription. A lot of non-protein-coding transcripts have a significant function in the translation process. Gene expressions depend on complex networks of diverse gene regulatory pathways. Several non-protein-coding RNAs regulate gene expression in a sequence-specific system either at the transcriptional level or post-transcriptional level. They include a significant part of the gene expression regulatory network. RNA-mediated gene regulation machinery is evolutionarily ancient. They well-evolved during the evolutionary time and are becoming much more complex than had been expected. In this review, we are trying to summarizing the current knowledge in the field of RNA-mediated gene silencing.

Identification of Small RNAs During High Light Acclimation in Arabidopsis thaliana

The biological significance of non-coding RNAs (ncRNAs) has been firmly established to be important for the regulation of genes involved in stress acclimation. Light plays an important role for the growth of plants providing the energy for photosynthesis; however, excessive light conditions can also cause substantial defects. Small RNAs (sRNAs) are a class of non-coding RNAs that regulate transcript levels of protein-coding genes and mediate epigenetic silencing. Next generation sequencing facilitates the identification of small non-coding RNA classes such as miRNAs (microRNAs) and small-interfering RNAs (siRNAs), and long non-coding RNAs (lncRNAs), but changes in the ncRNA transcriptome in response to high light are poorly understood. We subjected Arabidopsis plants to high light conditions and performed a temporal in-depth study of the transcriptome data after 3 h, 6 h, and 2 days of high light treatment. We identified a large number of high light responsive miRNAs and sRNAs derived from NAT gene pairs, lncRNAs and TAS transcripts. We performed target predictions for differentially expressed miRNAs and correlated their expression levels through mRNA sequencing data. GO analysis of the targets revealed an overrepresentation of genes involved in transcriptional regulation. In A. thaliana, sRNA-mediated regulation of gene expression in response to high light treatment is mainly carried out by miRNAs and sRNAs derived from NAT gene pairs, and from lncRNAs. This study provides a deeper understanding of sRNA-dependent regulatory networks in high light acclimation.

Genome-wide analysis of long non-coding RNAs responsive to multiple nutrient stresses in Arabidopsis thaliana

Nutrient stress is the most important environmental stress that limits plant growth and development. Although recent evidence highlights the vital functions of long non-coding RNAs (lncRNA) in response to single nutrient stress in some model plants, a comprehensive investigation of the effect of lncRNAs in response to nutrient stress has not been performed in Arabidopsis thaliana. Here, we presented the identification and characterization of lncRNAs under seven nutrient stress conditions. The expression pattern analysis revealed that aberrant expression of lncRNAs is a stress-specific manner under nutrient stress conditions and that lncRNAs are more sensitive to nutrient stress than protein-coding genes (PCGs). Moreover, competing endogenous RNA (ceRNA) network and lncRNA-mRNA co-expression network (CEN) were constructed to explore the potential function of these lncRNAs under nutrient stress conditions. We further combined different expressed lncRNAs with ceRNA network and CEN to select key lncRNAs in response to nutrient stress. Together, our study provides important information for further insights into the role of lncRNAs in response to stress in plants.

NATpare: a pipeline for high-throughput prediction and functional analysis of nat-siRNAs

Natural antisense transcript-derived small interfering RNAs (nat-siRNAs) are a class of functional small RNA (sRNA) that have been found in both plant and animals kingdoms. In plants, these sRNAs have been shown to suppress the translation of messenger RNAs (mRNAs) by directing the RNA-induced silencing complex (RISC) to their sequence-specific mRNA target(s). Current computational tools for classification of nat-siRNAs are limited in number and can be computationally infeasible to use. In addition, current methods do not provide any indication of the function of the predicted nat-siRNAs. Here, we present a new software pipeline, called NATpare, for prediction and functional analysis of nat-siRNAs using sRNA and degradome sequencing data. Based on our benchmarking in multiple plant species, NATpare substantially reduces the time required to perform prediction with minimal resource requirements allowing for comprehensive analysis of nat-siRNAs in larger and more complex organisms for the first time. We then exemplify the use of NATpare by identifying tissue and stress specific nat-siRNAs in multiple Arabidopsis thaliana datasets.

Identification of small non-coding RNAs responsive to GUN1 and GUN5 related retrograde signals in Arabidopsis thaliana

Chloroplast perturbations activate retrograde signalling pathways, causing dynamic changes of gene expression. Besides transcriptional control of gene expression, different classes of small non-coding RNAs (sRNAs) act in gene expression control, but comprehensive analyses regarding their role in retrograde signalling are lacking. We performed sRNA profiling in response to norflurazon (NF), which provokes retrograde signals, in Arabidopsis thaliana wild type (WT) and the two retrograde signalling mutants gun1 and gun5. The RNA samples were also used for mRNA and long non-coding RNA profiling to link altered sRNA levels to changes in the expression of their cognate target RNAs. We identified 122 sRNAs from all known sRNA classes that were responsive to NF in the WT. Strikingly, 142 and 213 sRNAs were found to be differentially regulated in both mutants, indicating a retrograde control of these sRNAs. Concomitant with the changes in sRNA expression, we detected about 1500 differentially expressed mRNAs in the NF-treated WT and around 900 and 1400 mRNAs that were differentially regulated in the gun1 and gun5 mutants, with a high proportion (~30%) of genes encoding plastid proteins. Furthermore, around 20% of predicted miRNA targets code for plastid-localised proteins. Among the sRNA-target pairs, we identified pairs with an anticorrelated expression as well pairs showing other expressional relations, pointing to a role of sRNAs in balancing transcriptional changes upon retrograde signals. Based on the comprehensive changes in sRNA expression, we assume a considerable impact of sRNAs in retrograde-dependent transcriptional changes to adjust plastidic and nuclear gene expression.

Identification of small RNAs during cold acclimation in Arabidopsis thaliana

BACKGROUND

Cold stress causes dynamic changes in gene expression that are partially caused by small non-coding RNAs since they regulate protein coding transcripts and act in epigenetic gene silencing pathways. Thus, a detailed analysis of transcriptional changes of small RNAs (sRNAs) belonging to all known sRNA classes such as microRNAs (miRNA) and small interfering RNA (siRNAs) in response to cold contributes to an understanding of cold-related transcriptome changes.

RESULT

We subjected A. thaliana plants to cold acclimation conditions (4 °C) and analyzed the sRNA transcriptomes after 3 h, 6 h and 2 d. We found 93 cold responsive differentially expressed miRNAs and only 14 of these were previously shown to be cold responsive. We performed miRNA target prediction for all differentially expressed miRNAs and a GO analysis revealed the overrepresentation of miRNA-targeted transcripts that code for proteins acting in transcriptional regulation. We also identified a large number of differentially expressed cis- and trans-nat-siRNAs, as well as sRNAs that are derived from long non-coding RNAs. By combining the results of sRNA and mRNA profiling with miRNA target predictions and publicly available information on transcription factors, we reconstructed a cold-specific, miRNA and transcription factor dependent gene regulatory network. We verified the validity of links in the network by testing its ability to predict target gene expression under cold acclimation.

CONCLUSION

In A. thaliana, miRNAs and sRNAs derived from cis- and trans-NAT gene pairs and sRNAs derived from lncRNAs play an important role in regulating gene expression in cold acclimation conditions. This study provides a fundamental database to deepen our knowledge and understanding of regulatory networks in cold acclimation.

Full-Length Transcript-Based Proteogenomics of Rice Improves Its Genome and Proteome Annotation

Rice (Oryza sativa) molecular breeding has gained considerable attention in recent years, but inaccurate genome annotation hampers its progress and functional studies of the rice genome. In this study, we applied single-molecule long-read RNA sequencing (lrRNA_seq)-based proteogenomics to reveal the complexity of the rice transcriptome and its coding abilities. Surprisingly, approximately 60% of loci identified by lrRNA_seq are associated with natural antisense transcripts (NATs). The high-density genomic arrangement of NAT genes suggests their potential roles in the multifaceted control of gene expression. In addition, a large number of fusion and intergenic transcripts have been observed. Furthermore, 906,456 transcript isoforms were identified, and 72.9% of the genes can generate splicing isoforms. A total of 706,075 posttranscriptional events were subsequently categorized into 10 subtypes, demonstrating the interdependence of posttranscriptional mechanisms that contribute to transcriptome diversity. Parallel short-read RNA sequencing indicated that lrRNA_seq has a superior capacity for the identification of longer transcripts. In addition, over 190,000 unique peptides belonging to 9,706 proteoforms/protein groups were identified, expanding the diversity of the rice proteome. Our findings indicate that the genome organization, transcriptome diversity, and coding potential of the rice transcriptome are far more complex than previously anticipated.

Transcriptome-wide-scale-predicted dsRNAs potentially involved in RNA homoeostasis are remarkably excluded from genes with no/very low expression in all developmental stages

RNA interference (RNAi) refers to a conserved posttranscriptional mechanism for the degradation of RNA by short dsRNAs. A genome-wide analysis of mRNAs that are complementary to RNAs of variable length that are transcribed from the full transcriptome and susceptible to being loaded onto Argonaute type 2 was performed through computational searches in the Drosophila model. We report the segments of RNAs that are complementary to mRNAs originating from introns, the exons of mRNAs and lncRNAs as a potential source of siRNAs. A full catalogue of the mRNAs that fulfill these criteria is presented, along with the quantification of multiple annealing. The catalogue was assessed for biological validation using three published lists: two for Ago2-associated RNAs and one for dsRNAs isolated from a crude extract. A broad spectrum of mRNAs were found to theoretically form intermolecular segmental dsRNAs, which should qualify them as Dicer/Ago2 substrates if they exist in vivo. These results suggest a genome-wide scale of mRNA homoeostasis via RNAi metabolism and could extend the known roles of canonical miRNAs and hairpin RNAs. The distribution of the genes for which transcripts are engaged in intermolecular segmental pairing is largely lacking in the gene collections defined as showing no expression in each individual developmental stage from early embryos to adulthood. This trend was also observed for the genes showing very low expression from the 8-12-hour embryonic to larval stage 2. This situation was also suggested by the 3 lists generated with minimal 20-, 25- and 30-base pairing lengths.

Full-length mRNA sequencing and gene expression profiling reveal broad involvement of natural antisense transcript gene pairs in pepper development and response to stresses

Pepper is an important vegetable with great economic value and unique biological features. In the past few years, significant development has been made toward understanding the huge complex pepper genome; however, pepper functional genomics has not been well studied. To better understand the pepper gene structure and pepper gene regulation, we conducted full-length mRNA sequencing by PacBio sequencing and obtained 57 862 high-quality full-length mRNA sequences derived from 18 362 previously annotated and 5769 newly detected genes. New gene models were built that combined the full-length mRNA sequences and corrected approximately 500 fragmented gene models from previous annotations. Based on the full-length mRNA, we identified 4114 and 5880 pepper genes forming natural antisense transcript (NAT) genes in-cis and in-trans, respectively. Most of these genes accumulate small RNAs in their overlapping regions. By analyzing these NAT gene expression patterns in our transcriptome data, we identified many NAT pairs responsive to a variety of biological processes in pepper. Pepper formate dehydrogenase 1 (FDH1), which is required for R-gene-mediated disease resistance, may be regulated by nat-siRNAs and participate in a positive feedback loop in salicylic acid biosynthesis during resistance responses. Several cis-NAT pairs and subgroups of trans-NAT genes were responsive to pepper pericarp and placenta development, which may play roles in capsanthin and capsaicin biosynthesis. Using a comparative genomics approach, the evolutionary mechanisms of cis-NATs were investigated, and we found that an increase in intergenic sequences accounted for the loss of most cis-NATs, while transposon insertion contributed to the formation of most new cis-NATs. OPEN RESEARCH BADGES: This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at http://bigd.big.ac.cn/gsa Accession number, CRA001412.

Prediction of regulatory long intergenic non-coding RNAs acting in trans through base-pairing interactions

Background

Long intergenic non-coding RNAs (lincRNAs) can act as regulators of expression of protein-coding genes. Trans-natural antisense transcripts (trans-NATs) are a type of lincRNAs that contain sequence complementary to mRNA from other loci. The regulatory potential of trans-NATs has been poorly studied in eukaryotes and no example of trans-NATs regulating gene expression in plants are reported. The goal of this study was to identify lincRNAs, and particularly trans-NATs, in Arabidopsis thaliana that have a potential to regulate expression of target genes in trans at the transcriptional or translational level.

Results

We identified 1001 lincRNAs using an RNAseq dataset from total polyA⁺ and polysome-associated RNA of seedlings grown under high and low phosphate, or shoots and roots treated with different phytohormones, of which 550 were differentially regulated. Approximately 30% of lincRNAs showed conservation amongst Brassicaceae and 25% harbored transposon element (TE) sequences. Gene co-expression network analysis highlighted a group of lincRNAs associated with the response of roots to low phosphate. A total of 129 trans-NATs were predicted, of which 88 were significantly differentially expressed under at least one pairwise comparison. Five trans-NATs showed a positive correlation between their expression and target mRNA steady-state levels, and three showed a negative correlation. Expression of four trans-NATs positively correlated with a change in target mRNA polysome association. The regulatory potential of these trans-NATs did not implicate miRNA mimics nor siRNAs. We also looked for lincRNAs that could regulate gene expression in trans by Watson-Crick DNA:RNA base pairing with target protein-encoding loci. We identified 100 and 81 with a positive or negative correlation, respectively, with steady-state level of their predicted target. The regulatory potential of one such candidate lincRNA harboring a SINE TE sequence was validated in a protoplast assay on three distinct genes containing homologous TE sequence in their promoters. Construction of networks highlighted other putative lincRNAs with multiple predicted target loci for which expression was positively correlated with target gene expression.

Conclusions

This study identified lincRNAs in Arabidopsis with potential in regulating target gene expression in trans by both RNA:RNA and RNA:DNA base pairing and highlights lincRNAs harboring TE sequences in such activity.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5946-0) contains supplementary material, which is available to authorized users.

Control of Cognate Sense mRNA Translation by cis-Natural Antisense RNAs

Cis-Natural Antisense Transcripts (cis-NATs), which overlap protein coding genes and are transcribed from the opposite DNA strand, constitute an important group of noncoding RNAs. Whereas several examples of cis-NATs regulating the expression of their cognate sense gene are known, most cis-NATs function by altering the steady-state level or structure of mRNA via changes in transcription, mRNA stability, or splicing, and very few cases involve the regulation of sense mRNA translation. This study was designed to systematically search for cis-NATs influencing cognate sense mRNA translation in Arabidopsis (Arabidopsis thaliana). Establishment of a pipeline relying on sequencing of total polyA⁺ and polysomal RNA from Arabidopsis grown under various conditions (i.e. nutrient deprivation and phytohormone treatments) allowed the identification of 14 cis-NATs whose expression correlated either positively or negatively with cognate sense mRNA translation. With use of a combination of cis-NAT stable over-expression in transgenic plants and transient expression in protoplasts, the impact of cis-NAT expression on mRNA translation was confirmed for 4 out of 5 tested cis-NAT:sense mRNA pairs. These results expand the number of cis-NATs known to regulate cognate sense mRNA translation and provide a foundation for future studies of their mode of action. Moreover, this study highlights the role of this class of noncoding RNAs in translation regulation.

Extensive nuclear reprogramming and endoreduplication in mature leaf during floral induction

BACKGROUND

The floral transition is a complex developmental event, fine-tuned by various environmental and endogenous cues to ensure the success of offspring production. Leaves are key organs in sensing floral inductive signals, such as a change in light regime, and in the production of the mobile florigen. CONSTANS and FLOWERING LOCUS T are major players in leaves in response to photoperiod. Morphological and molecular events during the floral transition have been intensively studied in the shoot apical meristem. To better understand the concomitant processes in leaves, which are less described, we investigated the nuclear changes in fully developed leaves during the time course of the floral transition.

RESULTS

We highlighted new putative regulatory candidates of flowering in leaves. We observed differential expression profiles of genes related to cellular, hormonal and metabolic actions, but also of genes encoding long non-coding RNAs and new natural antisense transcripts. In addition, we detected a significant increase in ploidy level during the floral transition, indicating endoreduplication.

CONCLUSIONS

Our data indicate that differentiated mature leaves, possess physiological plasticity and undergo extensive nuclear reprogramming during the floral transition. The dynamic events point at functionally related networks of transcription factors and novel regulatory motifs, but also complex hormonal and metabolic changes.

Recent advances in small RNA mediated plant-virus interactions

Small RNAs (sRNA) are reported to play pivotal roles in the epigenetic and post-transcriptional regulation of gene expression during growth, development, and stress response in plants. Recently, the involvement of two different classes of sRNAs namely, miRNAs (microRNAs), and siRNAs (small interfering RNAs) in biotic stress response has been underlined. Notably, during virus infection, these sRNAs deploy antiviral defense by regulating the gene expression of the modulators of host defense pathways. As a counter defense, viruses have evolved strategic pathways involving the production of suppressors that interfere with the host silencing machinery. This molecular arms race between the sophisticated gene regulatory mechanism of host plants fine-tuned by sRNAs and the defense response exhibited by the virus has gained much attention among the researchers. So far, several reports have been published showing the mechanistic insights on sRNA-regulated defense mechanism in response to virus infection in several crop plants. In this context, our review enumerates the molecular mechanisms underlying host immunity against viruses mediated by sRNAs, the counter defense strategies employed by viruses to surpass this immunogenic response and the advances made in our understanding of plant-virus interactions. Altogether, the report would be insightful for the researchers working to decode the sRNA-mediated defense response in crop plants challenged with virus infection.

Identification of a novel antisense RNA that regulates growth hormone receptor expression in chickens

Natural antisense transcripts (NATs) are widely present in mammalian genomes and act as pivotal regulator molecules of gene expression. However, studies on NATs in the chicken are relatively rare. We identified a novel antisense transcript in the chicken, designated GHR-AS-EST, transcribed from the growth hormone receptor (GHR) locus, which encodes a well-known regulatory molecule of muscle development and fat deposition. GHR-AS-EST is predominantly expressed in the chicken liver and muscle tissues. GHR-AS-EST sequence conservation among vertebrates is weak. GHR-AS-EST forms an RNA-RNA duplex with GHBP to increase its stability, and regulates the expression of GHR sense transcripts at both the mRNA and protein levels. Further, GHR-AS-EST promotes cell proliferation by stimulating the expression of signaling factors in the JAK2/STAT pathway, and contributes to fat deposition via downregulating the expression of signaling factors in the JAK2/SOCS pathway in LMH hepatocellular carcinoma cells. We expect that the discovery of a NAT for a regulatory gene associated with cell proliferation and lipolysis will further our understanding of the molecular regulation of both muscle development and fat deposition.

Defective XRN3-mediated transcription termination in Arabidopsis affects the expression of protein-coding genes

Arabidopsis thaliana contains two nuclear XRN2/3 5'-3' exonucleases that are homologs of yeast and human Rat1/Xrn2 proteins involved in the processing and degradation of several classes of nuclear RNAs and in transcription termination of RNA polymerase II. Using strand-specific short read sequencing we show that knockdown of XRN3 leads to an altered expression of hundreds of genes and the accumulation of uncapped and polyadenylated read-through transcripts generated by inefficiently terminated Pol II. Our data support the notion that XRN3-mediated changes in the expression of a subset of genes are caused by upstream read-through transcription and these effects are enhanced by RNA-mRNA chimeras generated in xrn3 plants. In turn, read-through transcripts that are antisense to downstream genes may trigger production of siRNA. Our results highlight the importance of XRN3 exoribonuclease in Pol II transcription termination in plants and show that disturbance in this process may significantly alter gene expression.

Bioinformatic analysis of regulation of natural antisense transcripts by transposable elements in human mRNA

Non-coding RNA is no longer considered to be "junk" DNA, based on evidence uncovered in recent decades. In particular, the important role played by natural antisense transcripts (NATs) in regulating the expression of genes is receiving increasing attention. However, the regulatory mechanisms of NATs remain incompletely understood. It is well-known that the insertion of transposable elements (TEs) can affect gene transcription. Using a bioinformatics approach, we identified NATs using human mRNA sequences from the UCSC Genome Browser Database. Our in silico analysis identified 1079 NATs and 700 sense-antisense gene pairs. We identified 179 NATs that showed evidence of having been affected by TEs during cellular gene expression. These findings may provide an understanding of the complex regulation mechanisms of NATs. If our understanding of NATs as modulators of gene expression is further enhanced, we can develop ways to control gene expression.

Natural antisense transcripts drive a regulatory cascade controlling c-MYC transcription

Cis-natural antisense transcripts (cis-NATs) are long noncoding RNAs transcribed from the opposite strand and overlapping coding and noncoding genes on the sense strand. cis-NATs are widely present in the human genome and can be involved in multiple mechanisms of gene regulation. Here, we describe the presence of cis-NATs in the 3′ distal region of the c-MYC locus and investigate their impact on transcriptional regulation of this key oncogene in human cancers. We found that cis-NATs are produced as consequence of the activation of cryptic transcription initiation sites in the 3′ distal region downstream of the c-MYC 3′UTR. The process is tightly regulated and leads to the formation of two main transcripts, NAT6531 and NAT6558, which differ in their ability to fold into stem-loop secondary structures. NAT6531 acts as a substrate for DICER and as a source of small RNAs capable of modulating c-MYC transcription. This complex system, based on the interplay between cis-NATs and NAT-derived small RNAs, may represent an important layer of epigenetic regulation of the expression of c-MYC and other genes in human cells.

Non-coding RNAs and Their Roles in Stress Response in Plants

Eukaryotic genomes encode thousands of non-coding RNAs (ncRNAs), which play crucial roles in transcriptional and post-transcriptional regulation of gene expression. Accumulating evidence indicates that ncRNAs, especially microRNAs (miRNAs) and long ncRNAs (lncRNAs), have emerged as key regulatory molecules in plant stress responses. In this review, we have summarized the current progress on the understanding of plant miRNA and lncRNA identification, characteristics, bioinformatics tools, and resources, and provided examples of mechanisms of miRNA- and lncRNA-mediated plant stress tolerance.

Natural antisense transcripts are significantly involved in regulation of drought stress in maize

Natural antisense transcripts (NATs) are a prominent and complex class of regulatory RNAs. Using strand-specific RNA sequencing, we identified 1769 sense and antisense transcript pairs (NAT pairs) in two maize inbreds with different sensitivity to drought, as well as in two derivative recombination inbred lines (RILs). A significantly higher proportion of NATs relative to non-NATs are specifically expressed under water stress (WS). Surprisingly, expression of sense and antisense transcripts produced by NAT pairs is significantly correlated, particularly under WS. We found an unexpected large proportion of NATs with protein coding potential, as estimated by ribosome release scores. Small RNAs significantly accumulate within NAT pairs, with 21 nt smRNA particularly enriched in overlapping regions of these pairs of genes. The abundance of these smRNAs is significantly altered in the leafbladeless1 mutant, suggesting that these genes may be regulated by the tasiRNA pathway. Further, NATs are significantly hypomethylated and include fewer transposable element sequences relative to non-NAT genes. NAT gene regions also exhibit higher levels of H3K36me3, H3K9ac, and H3K4me3, but lower levels of H3K27me3, indicating that NAT gene pairs generally exhibit an open chromatin configuration. Finally, NAT pairs in 368 diverse maize inbreds and 19 segregating populations were specifically enriched for polymorphisms associated with drought tolerance. Taken together, the data highlight the potential impact of that small RNAs and histone modifications have in regulation of NAT expression, and the significance of NATs in response to WS.

Plant organ evolution revealed by phylotranscriptomics in Arabidopsis thaliana

The evolution of phenotypes occurs through changes both in protein sequence and gene expression levels. Though much of plant morphological evolution can be explained by changes in gene expression, examining its evolution has challenges. To gain a new perspective on organ evolution in plants, we applied a phylotranscriptomics approach. We combined a phylostratigraphic approach with gene expression based on the strand-specific RNA-seq data from seedling, floral bud, and root of 19 Arabidopsis thaliana accessions to examine the age and sequence divergence of transcriptomes from these organs and how they adapted over time. Our results indicate that, among the sense and antisense transcriptomes of these organs, the sense transcriptomes of seedlings are the evolutionarily oldest across all accessions and are the most conserved in amino acid sequence for most accessions. In contrast, among the sense transcriptomes from these same organs, those from floral bud are evolutionarily youngest and least conserved in sequence for most accessions. Different organs have adaptive peaks at different stages in their evolutionary history; however, all three show a common adaptive signal from the Magnoliophyta to Brassicale stage. Our research highlights how phylotranscriptomic analyses can be used to trace organ evolution in the deep history of plant species.

Evolution of RNA- and DNA-guided antivirus defense systems in prokaryotes and eukaryotes: common ancestry vs convergence

Abstract

Complementarity between nucleic acid molecules is central to biological information transfer processes. Apart from the basal processes of replication, transcription and translation, complementarity is also employed by multiple defense and regulatory systems. All cellular life forms possess defense systems against viruses and mobile genetic elements, and in most of them some of the defense mechanisms involve small guide RNAs or DNAs that recognize parasite genomes and trigger their inactivation. The nucleic acid-guided defense systems include prokaryotic Argonaute (pAgo)-centered innate immunity and CRISPR-Cas adaptive immunity as well as diverse branches of RNA interference (RNAi) in eukaryotes. The archaeal pAgo machinery is the direct ancestor of eukaryotic RNAi that, however, acquired additional components, such as Dicer, and enormously diversified through multiple duplications. In contrast, eukaryotes lack any heritage of the CRISPR-Cas systems, conceivably, due to the cellular toxicity of some Cas proteins that would get activated as a result of operon disruption in eukaryotes. The adaptive immunity function in eukaryotes is taken over partly by the PIWI RNA branch of RNAi and partly by protein-based immunity. In this review, I briefly discuss the interplay between homology and analogy in the evolution of RNA- and DNA-guided immunity, and attempt to formulate some general evolutionary principles for this ancient class of defense systems.

Reviewers

This article was reviewed by Mikhail Gelfand and Bojan Zagrovic.

History, Discovery, and Classification of lncRNAs

The RNA World Hypothesis suggests that prebiotic life revolved around RNA instead of DNA and proteins. Although modern cells have changed significantly in 4 billion years, RNA has maintained its central role in cell biology. Since the discovery of DNA at the end of the nineteenth century, RNA has been extensively studied. Many discoveries such as housekeeping RNAs (rRNA, tRNA, etc.) supported the messenger RNA model that is the pillar of the central dogma of molecular biology, which was first devised in the late 1950s. Thirty years later, the first regulatory non-coding RNAs (ncRNAs) were initially identified in bacteria and then in most eukaryotic organisms. A few long ncRNAs (lncRNAs) such as H19 and Xist were characterized in the pre-genomic era but remained exceptions until the early 2000s. Indeed, when the sequence of the human genome was published in 2001, studies showed that only about 1.2% encodes proteins, the rest being deemed "non-coding." It was later shown that the genome is pervasively transcribed into many ncRNAs, but their functionality remained controversial. Since then, regulatory lncRNAs have been characterized in many species and were shown to be involved in processes such as development and pathologies, revealing a new layer of regulation in eukaryotic cells. This newly found focus on lncRNAs, together with the advent of high-throughput sequencing, was accompanied by the rapid discovery of many novel transcripts which were further characterized and classified according to specific transcript traits.In this review, we will discuss the many discoveries that led to the study of lncRNAs, from Friedrich Miescher's "nuclein" in 1869 to the elucidation of the human genome and transcriptome in the early 2000s. We will then focus on the biological relevance during lncRNA evolution and describe their basic features as genes and transcripts. Finally, we will present a non-exhaustive catalogue of lncRNA classes, thus illustrating the vast complexity of eukaryotic transcriptomes.

Cis-Natural Antisense Transcripts Are Mainly Co-expressed with Their Sense Transcripts and Primarily Related to Energy Metabolic Pathways during Muscle Development

Cis-natural antisense transcripts (cis-NATs) are a new class of RNAs identified in various species. However, the biological functions of cis-NATs are largely unknown. In this study, we investigated the transcriptional characteristics and functions of cis-NATs in the muscle tissue of lean Landrace and indigenous fatty Lantang pigs. In total, 3,306 cis-NATs of 2,469 annotated genes were identified in the muscle tissue of pigs. More than 1,300 cis-NATs correlated with their sense genes at the transcriptional level, and approximately 80% of them were co-expressed in the two breeds. Furthermore, over 1,200 differentially expressed cis-NATs were identified during muscle development. Function annotation showed that the cis-NATs participated in muscle development mainly by co-expressing with genes involved in energy metabolic pathways, including citrate cycle (TCA cycle), glycolysis or gluconeogenesis, mitochondrial activation and so on. Moreover, these cis-NATs and their sense genes abruptly increased at the transition from the late fetal stages to the early postnatal stages and then decreased along with muscle development. In conclusion, the cis-NATs in the muscle tissue of pigs were identified and determined to be mainly co-expressed with their sense genes. The co-expressed cis-NATs and their sense gene were primarily related to energy metabolic pathways during muscle development in pigs. Our results offered novel evidence on the roles of cis-NATs during the muscle development of pigs.

Small interfering RNAs from bidirectional transcripts of GhMML3_A12 regulate cotton fiber development

Natural antisense transcripts (NATs) are commonly observed in eukaryotic genomes, but only a limited number of such genes have been identified as being involved in gene regulation in plants. In this research, we investigated the function of small RNA derived from a NAT in fiber cell development. Using a map-based cloning strategy for the first time in tetraploid cotton, we cloned a naked seed mutant gene (N1 ) encoding a MYBMIXTA-like transcription factor 3 (MML3)/GhMYB25-like in chromosome A12, GhMML3_A12, that is associated with fuzz fiber development. The extremely low expression of GhMML3_A12 in N1 is associated with NAT production, driven by its 3' antisense promoter, as indicated by the promoter-driven histochemical staining assay. In addition, small RNA deep sequencing analysis suggested that the bidirectional transcriptions of GhMML3_A12 form double-stranded RNAs and generate 21-22 nt small RNAs. Therefore, in a fiber-specific manner, small RNA derived from the GhMML3_A12 locus can mediate GhMML3_A12 mRNA self-cleavage and result in the production of naked seeds followed by lint fiber inhibition in N1 plants. The present research reports the first observation of gene-mediated NATs and siRNA directly controlling fiber development in cotton.

Strand-Specific RNA-Seq Analyses of Fruiting Body Development in Coprinopsis cinerea

The basidiomycete fungus Coprinopsis cinerea is an important model system for multicellular development. Fruiting bodies of C. cinerea are typical mushrooms, which can be produced synchronously on defined media in the laboratory. To investigate the transcriptome in detail during fruiting body development, high-throughput sequencing (RNA-seq) was performed using cDNA libraries strand-specifically constructed from 13 points (stages/tissues) with two biological replicates. The reads were aligned to 14,245 predicted transcripts, and counted for forward and reverse transcripts. Differentially expressed genes (DEGs) between two adjacent points and between vegetative mycelium and each point were detected by Tag Count Comparison (TCC). To validate RNA-seq data, expression levels of selected genes were compared using RPKM values in RNA-seq data and qRT-PCR data, and DEGs detected in microarray data were examined in MA plots of RNA-seq data by TCC. We discuss events deduced from GO analysis of DEGs. In addition, we uncovered both transcription factor candidates and antisense transcripts that are likely to be involved in developmental regulation for fruiting.

Post-transcriptional gene silencing triggered by sense transgenes involves uncapped antisense RNA and differs from silencing intentionally triggered by antisense transgenes

Although post-transcriptional gene silencing (PTGS) has been studied for more than a decade, there is still a gap in our understanding of how de novo silencing is initiated against genetic elements that are not supposed to produce double-stranded (ds)RNA. Given the pervasive transcription occurring throughout eukaryote genomes, we tested the hypothesis that unintended transcription could produce antisense (as)RNA molecules that participate to the initiation of PTGS triggered by sense transgenes (S-PTGS). Our results reveal a higher level of asRNA in Arabidopsis thaliana lines that spontaneously trigger S-PTGS than in lines that do not. However, PTGS triggered by antisense transgenes (AS-PTGS) differs from S-PTGS. In particular, a hypomorphic ago1 mutation that suppresses S-PTGS prevents the degradation of asRNA but not sense RNA during AS-PTGS, suggesting a different treatment of coding and non-coding RNA by AGO1, likely because of AGO1 association to polysomes. Moreover, the intended asRNA produced during AS-PTGS is capped whereas the asRNA produced during S-PTGS derives from 3′ maturation of a read-through transcript and is uncapped. Thus, we propose that uncapped asRNA corresponds to the aberrant RNA molecule that is converted to dsRNA by RNA-DEPENDENT RNA POLYMERASE 6 in siRNA-bodies to initiate S-PTGS, whereas capped asRNA must anneal with sense RNA to produce dsRNA that initiate AS-PTGS.