RNA helicase A (RHA) is a highly conserved DEAD-box protein that activates transcription, modulates RNA splicing and binds the nuclear pore complex. The life cycle of typical mRNA involves RNA processing and translation after ribosome scanning of a relatively unstructured 5' untranslated region (UTR). The precursor RNAs of retroviruses and selected cellular genes harbor a complex 5' UTR and use a yet-to-be-identified host post-transcriptional effector to stimulate efficient translation. Here we show that RHA recognizes a structured 5'-terminal post-transcriptional control element (PCE) of a retrovirus and the JUND growth-control gene. RHA interacts with PCE RNA in the nucleus and cytoplasm, facilitates polyribosome association and is necessary for its efficient translation. Our results reveal a previously unidentified role for RHA in translation and implicate RHA as an integrative effector in the continuum of gene expression from transcription to translation.

Leaf senescence is an essential part of the plant lifecycle during which nutrients are re-allocated to other tissues. The regulation of leaf senescence is a complex process. However, the underlying mechanism is poorly understood. Here, we uncovered a novel and the pivotal role of Arabidopsis HDA9 (a RPD3-like histone deacetylase) in promoting the onset of leaf senescence. We found that HDA9 acts in complex with a SANT domain-containing protein POWERDRESS (PWR) and transcription factor WRKY53. Our genome-wide profiling of HDA9 occupancy reveals that HDA9 directly binds to the promoters of key negative regulators of senescence and this association requires PWR. Furthermore, we found that PWR is important for HDA9 nuclear accumulation. This study reveals an uncharacterized epigenetic complex involved in leaf senescence and provides mechanistic insights into how a histone deacetylase along with a chromatin-binding protein contribute to a robust regulatory network to modulate the onset of plant aging.DOI: http://dx.doi.org/10.7554/eLife.17214.001

RNA-templated RNA replication is essential for viral or viroid infectionAccording to the "RNA world" scenario, the appearance of RNA molecules simultaneously capable of self-replication and information storage signaled a major milestone in the evolution of life (14,24,25). In modern biology, RNA replication is central to viral or viroid infection, as well as to the regulation of cellular gene expression. Virus-encoded RNA-dependent RNA polymerases play a major role in the replication of RNA viruses (10, 23). Cellular RNA-dependent RNA polymerases generate double-stranded RNAs as triggers for RNA silencing (1). Intriguingly, the DNA-dependent cellular RNA polymerases can also transcribe at least two types of RNA templates: viroid RNAs (11,15,59) and the human hepatitis delta virus (HDV) RNA (28, 60). The replication of viroid and HDV RNAs raises the question of whether the DNA-templated transcription machinery also replicates other cellular RNAs yet to be identified. Elucidating the replication mechanisms of these infectious RNAs should help address this question of profound biological interest.Viroids are the smallest known nucleic acid-based infectious agents and self-replicating genetic units. Their "genomes" consist of single-stranded, circular RNAs ranging in size from 250 to 400 nucleotides (23). Viroids can replicate and spread throughout an infected plant, although they do not encode proteins, do not have encapsidation mechanisms, and do not require helper viruses. Furthermore, they cause devastating diseases by altering host gene expression and developmental processes (11,59). Evidently, viroid RNA genomes contain all of the sequence and structural information needed to mediate or trigger the various functions associated with infection. Potato spindle tuber viroid (PSTVd) is the type member of the family Pospiviroidae (8, 12). The PSTVd genome consists of 359 nucleotides and assumes a rod-shaped secondary structure in the native state (48) with five structural domains, as shown in Fig. 1A (26). This secondary structure, which is typical of viroids in the family Pospiviroidae, comprises many loops and bulges flanked by short Watson-Crick helices. Formation of this secondary structure is necessary for infection (62). During asymmetric rolling-circle replication of PSTVd (5), the plus circular strands serve as templates for the synthesis of concatemeric, linear minus strands, which then function as the replication intermediates for the synthesis of concatemeric, linear plus strands. These are subsequently cleaved into monomers and ligated into circular molecules (Fig. 1B). Without encoding proteins, PSTVd replicates in the nucleus of a host cell and

The ability of cells to perceive and translate versatile cues into differential chromatin and transcriptional states is critical for many biological processes. In plants, timely transition to a flowering state is crucial for successful reproduction. EARLY BOLTING IN SHORT DAY (EBS) is a negative transcriptional regulator that prevents premature flowering in Arabidopsis thaliana. We found that EBS contains bivalent bromo-adjacent homology (BAH)-plant homeodomain (PHD) reader modules that bind H3K27me3 and H3K4me3, respectively. We observed co-enrichment of a subset of EBS-associated genes with H3K4me3, H3K27me3, and Polycomb repressor complex 2 (PRC2). Notably, EBS adopted an autoinhibition mode to mediate its switch in binding preference between H3K27me3 and H3K4me3. This binding balance was critical because disruption of either EBS-H3K27me3 or EBS-H3K4me3 interaction induced early floral transition. Our results identify a bivalent chromatin reader capable of recognizing two antagonistic histone marks, and we propose a distinct mechanism of interaction between active and repressive chromatin states.