A nitric oxide regulated small RNA controls expression of genes involved in redox homeostasis in Bacillus subtilis.

RsaE is the only known trans-acting small regulatory RNA (sRNA) besides the ubiquitous 6S RNA that is conserved between the human pathogen Staphylococcus aureus and the soil-dwelling Firmicute Bacillus subtilis. Although a number of RsaE targets are known in S. aureus, neither the environmental signals that lead to its expression nor its physiological role are known. Here we show that expression of the B. subtilis homolog of RsaE is regulated by the presence of nitric oxide (NO) in the cellular milieu. Control of expression by NO is dependent on the ResDE two-component system in B. subtilis and we determined that the same is true in S. aureus. Transcriptome and proteome analyses revealed that many genes with functions related to oxidative stress and oxidation-reduction reactions were up-regulated in a B. subtilis strain lacking this sRNA. We have thus renamed it RoxS. The prediction of RoxS-dependent mRNA targets also suggested a significant enrichment for mRNAs related to respiration and electron transfer. Among the potential direct mRNA targets, we have validated the ppnKB mRNA, encoding an NAD+/NADH kinase, both in vivo and in vitro. RoxS controls both translation initiation and the stability of this transcript, in the latter case via two independent pathways implicating RNase Y and RNase III. Furthermore, RNase Y intervenes at an additional level by processing the 5' end of the RoxS sRNA removing about 20 nucleotides. Processing of RoxS allows it to interact more efficiently with a second target, the sucCD mRNA, encoding succinyl-CoA synthase, thus expanding the repertoire of targets recognized by this sRNA.

The ELF4-ELF3-LUX complex links the circadian clock to diurnal control of hypocotyl growth.

The circadian clock is required for adaptive responses to daily and seasonal changes in environmental conditions. Light and the circadian clock interact to consolidate the phase of hypocotyl cell elongation to peak at dawn under diurnal cycles in Arabidopsis thaliana. Here we identify a protein complex (called the evening complex)--composed of the proteins encoded by EARLY FLOWERING 3 (ELF3), ELF4 and the transcription-factor-encoding gene LUX ARRHYTHMO (LUX; also known as PHYTOCLOCK 1)--that directly regulates plant growth. ELF3 is both necessary and sufficient to form a complex between ELF4 and LUX, and the complex is diurnally regulated, peaking at dusk. ELF3, ELF4 and LUX are required for the proper expression of the growth-promoting transcription factors encoded by PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 (also known as PHYTOCHROME INTERACTING FACTOR 3-LIKE 6) under diurnal conditions. LUX targets the complex to the promoters of PIF4 and PIF5 in vivo. Mutations in PIF4 and/or PIF5 are epistatic to the loss of the ELF4-ELF3-LUX complex, suggesting that regulation of PIF4 and PIF5 is a crucial function of the complex. Therefore, the evening complex underlies the molecular basis for circadian gating of hypocotyl growth in the early evening.

Two novel members of the LhrC family of small RNAs in Listeria monocytogenes with overlapping regulatory functions but distinctive expression profiles.

Multicopy small RNAs (sRNAs) have gained recognition as an important feature of bacterial gene regulation. In the human pathogen Listeria monocytogenes, 5 homologous sRNAs, called LhrC1-5, control gene expression by base pairing to target mRNAs though 3 conserved UCCC motifs common to all 5 LhrCs. We show here that the sRNAs Rli22 and Rli33-1 are structurally and functionally related to LhrC1-5, expanding the LhrC family to 7 members, which makes it the largest multicopy sRNA family reported so far. Rli22 and Rli33-1 both contain 2 UCCC motifs important for post-transcriptional repression of 3 LhrC target genes. One such target, oppA, encodes a virulence-associated oligo-peptide binding protein. Like LhrC1-5, Rli22 and Rli33-1 employ their UCCC motifs to recognize the Shine-Dalgarno region of oppA mRNA and prevent formation of the ribosomal complex, demonstrating that the 7 sRNAs act in a functionally redundant manner. However, differential expression profiles of the sRNAs under infection-relevant conditions suggest that they might also possess non-overlapping functions. Collectively, this makes the LhrC family a unique case for studying the purpose of sRNA multiplicity in the context of bacterial virulence.

Various checkpoints prevent the synthesis of Staphylococcus aureus peptidoglycan hydrolase LytM in the stationary growth phase.

In Staphylococcus aureus, peptidoglycan metabolism plays a role in the host inflammatory response and pathogenesis. Transcription of the peptidoglycan hydrolases is activated by the essential 2-component system WalKR at low cell density. During stationary growth phase, WalKR is not active and transcription of the peptidoglycan hydrolase genes is repressed. In this work, we studied regulation of expression of the glycylglycine endopeptidase LytM. We show that, in addition to the transcriptional regulation mediated by WalKR, the synthesis of LytM is negatively controlled by a unique mechanism at the stationary growth phase. We have identified 2 different mRNAs encoding lytM, which vary in the length of their 5' untranslated (5'UTR) regions. LytM is predominantly produced from the WalKR-regulated mRNA transcript carrying a short 5'UTR. The lytM mRNA is also transcribed as part of a polycistronic operon with the upstream SA0264 gene and is constitutively expressed. Although SA0264 protein can be synthesized from the longer operon transcript, lytM cannot be translated because its ribosome-binding site is sequestered into a translationally inactive secondary structure. In addition, the effector of the agr system, RNAIII, can inhibit translation of lytM present on the operon without altering the transcript level but does not have an effect on the translation of the upstream gene. We propose that this dual regulation of lytM expression, at the transcriptional and post-transcriptional levels, contributes to prevent cell wall damage during the stationary phase of growth.

Global regulatory functions of the Staphylococcus aureus endoribonuclease III in gene expression.

RNA turnover plays an important role in both virulence and adaptation to stress in the Gram-positive human pathogen Staphylococcus aureus. However, the molecular players and mechanisms involved in these processes are poorly understood. Here, we explored the functions of S. aureus endoribonuclease III (RNase III), a member of the ubiquitous family of double-strand-specific endoribonucleases. To define genomic transcripts that are bound and processed by RNase III, we performed deep sequencing on cDNA libraries generated from RNAs that were co-immunoprecipitated with wild-type RNase III or two different cleavage-defective mutant variants in vivo. Several newly identified RNase III targets were validated by independent experimental methods. We identified various classes of structured RNAs as RNase III substrates and demonstrated that this enzyme is involved in the maturation of rRNAs and tRNAs, regulates the turnover of mRNAs and non-coding RNAs, and autoregulates its synthesis by cleaving within the coding region of its own mRNA. Moreover, we identified a positive effect of RNase III on protein synthesis based on novel mechanisms. RNase III-mediated cleavage in the 5' untranslated region (5'UTR) enhanced the stability and translation of cspA mRNA, which encodes the major cold-shock protein. Furthermore, RNase III cleaved overlapping 5'UTRs of divergently transcribed genes to generate leaderless mRNAs, which constitutes a novel way to co-regulate neighboring genes. In agreement with recent findings, low abundance antisense RNAs covering 44% of the annotated genes were captured by co-immunoprecipitation with RNase III mutant proteins. Thus, in addition to gene regulation, RNase III is associated with RNA quality control of pervasive transcription. Overall, this study illustrates the complexity of post-transcriptional regulation mediated by RNase III.

In vivo mapping of RNA-RNA interactions in Staphylococcus aureus using the endoribonuclease III.

Ribonucleases play key roles in gene regulation and in the expression of virulence factors in Staphylococcus aureus. Among these enzymes, the double-strand specific endoribonuclease III (RNase III) is a key mediator of mRNA processing and degradation. Recently, we have defined, direct target sites for RNase III processing on a genome-wide scale in S. aureus. Our approach is based on deep sequencing of cDNA libraries obtained from RNAs isolated by in vivo co-immunoprecipitation with wild-type RNase III and two cleavage-defective mutants. The use of such catalytically inactivated enzymes, which still retain their RNA binding capacity, allows the identification of novel RNA substrates of RNase III. In this report, we will summarize the diversity of RNase III functions, discuss the advantages and the limitations of the approach, and how this strategy identifies novel mRNA targets of small non-coding RNAs in S. aureus.

PhenoWell®-A novel screening system for soil-grown plants.

As agricultural production is reaching its limits regarding outputs and land use, the need to further improve crop yield is greater than ever. The limited translatability from in vitro lab results into more natural growth conditions in soil remains problematic. Although considerable progress has been made in developing soil-growth assays to tackle this bottleneck, the majority of these assays use pots or whole trays, making them not only space- and resource-intensive, but also hampering the individual treatment of plants. Therefore, we developed a flexible and compact screening system named PhenoWell® in which individual seedlings are grown in wells filled with soil allowing single-plant treatments. The system makes use of an automated image-analysis pipeline that extracts multiple growth parameters from individual seedlings over time, including projected rosette area, relative growth rate, compactness, and stockiness. Macronutrient, hormone, salt, osmotic, and drought stress treatments were tested in the PhenoWell® system. The system is also optimized for maize with results that are consistent with Arabidopsis while different in amplitude. We conclude that the PhenoWell® system enables a high-throughput, precise, and uniform application of a small amount of solution to individually soil-grown plants, which increases the replicability and reduces variability and compound usage.

Probing mRNA structure and sRNA-mRNA interactions in bacteria using enzymes and lead(II).

Enzymatic probing and lead(II)-induced cleavages have been developed to study the secondary structure of RNA molecules either free or engaged in complex with different ligands. Using a combination of probes with different specificities (unpaired vs. paired regions), it is possible to get information on the accessibility of each nucleotide, on the binding site of a ligand (noncoding RNAs, protein, metabolites), and on RNA conformational changes that accompanied ligand binding or environmental conditions (temperature, pH, ions, etc.). The detection of the cleavages can be conducted by two different ways, which are chosen according to the length of the studied RNA. The first method uses end-labeled RNA molecules and the second one involves primer extension by reverse transcriptase. We provide here an experimental procedure that was designed to map the structure of mRNA and mRNA-sRNA interaction in vitro.

sRNA-mediated activation of gene expression by inhibition of 5'-3' exonucleolytic mRNA degradation.

Post-transcriptional control by small regulatory RNA (sRNA) is critical for rapid adaptive processes. sRNAs can directly modulate mRNA degradation in Proteobacteria without interfering with translation. However, Firmicutes have a fundamentally different set of ribonucleases for mRNA degradation and whether sRNAs can regulate the activity of these enzymes is an open question. We show that Bacillus subtilis RoxS, a major trans-acting sRNA shared with Staphylococus aureus, prevents degradation of the yflS mRNA, encoding a malate transporter. In the presence of malate, RoxS transiently escapes from repression by the NADH-sensitive transcription factor Rex and binds to the extreme 5'-end of yflS mRNA. This impairs the 5'-3' exoribonuclease activity of RNase J1, increasing the half-life of the primary transcript and concomitantly enhancing ribosome binding to increase expression of the transporter. Globally, the different targets regulated by RoxS suggest that it helps readjust the cellular NAD+/NADH balance when perturbed by different stimuli.

Nucleolin Promotes Heat Shock-Associated Translation of VEGF-D to Promote Tumor Lymphangiogenesis.

The vascular endothelial growth factor VEGF-D promotes metastasis by inducing lymphangiogenesis and dilatation of the lymphatic vasculature, facilitating tumor cell extravasion. Here we report a novel level of control for VEGF-D expression at the level of protein translation. In human tumor cells, VEGF-D colocalized with eIF4GI and 4E-BP1, which can program increased initiation at IRES motifs on mRNA by the translational initiation complex. In murine tumors, the steady-state level of VEGF-D protein was increased despite the overexpression and dephosphorylation of 4E-BP1, which downregulates protein synthesis, suggesting the presence of an internal ribosome entry site (IRES) in the 5' UTR of VEGF-D mRNA. We found that nucleolin, a nucleolar protein involved in ribosomal maturation, bound directly to the 5'UTR of VEGF-D mRNA, thereby improving its translation following heat shock stress via IRES activation. Nucleolin blockade by RNAi-mediated silencing or pharmacologic inhibition reduced VEGF-D translation along with a subsequent constriction of lymphatic vessels in tumors. Our results identify nucleolin as a key regulator of VEGF-D expression, deepening understanding of lymphangiogenesis control during tumor formation. Cancer Res; 76(15); 4394-405. ©2016 AACR.

A method to map changes in bacterial surface composition induced by regulatory RNAs in Escherichia coli and Staphylococcus aureus.

We have adapted a method to map cell surface proteins and to monitor the effect of specific regulatory RNAs on the surface composition of the bacteria. This method involves direct labeling of surface proteins of living bacteria using fluorescent dyes and a subsequent separation of the crude extract by 2D gel electrophoresis. The strategy yields a substantial enrichment in surface proteins over cytoplasmic proteins. We validated this method by monitoring the effect of the regulatory RNA MicA in Escherichia coli, which regulates the synthesis of several outer membrane proteins, and highlighted the role of Staphylococcus aureus RNAIII for the maintenance of cell wall integrity.

Hypoxia induces VEGF-C expression in metastatic tumor cells via a HIF-1α-independent translation-mediated mechanism.

Various tumors metastasize via lymph vessels and lymph nodes to distant organs. Even though tumors are hypoxic, the mechanisms of how hypoxia regulates lymphangiogenesis remain poorly characterized. Here, we show that hypoxia reduced vascular endothelial growth factor C (VEGF-C) transcription and cap-dependent translation via the upregulation of hypophosphorylated 4E-binding protein 1 (4E-BP1). However, initiation of VEGF-C translation was induced by hypoxia through an internal ribosome entry site (IRES)-dependent mechanism. IRES-dependent VEGF-C translation was independent of hypoxia-inducible factor 1α (HIF-1α) signaling. Notably, the VEGF-C IRES activity was higher in metastasizing tumor cells in lymph nodes than in primary tumors, most likely because lymph vessels in these lymph nodes were severely hypoxic. Overall, this transcription-independent but translation-dependent upregulation of VEGF-C in hypoxia stimulates lymphangiogenesis in tumors and lymph nodes and may contribute to lymphatic metastasis.

ELF3 recruitment to the PRR9 promoter requires other Evening Complex members in the Arabidopsis circadian clock.

Biological timekeeping is essential for proper growth and development. Organisms such as the model plant Arabidopsis use the circadian clock to coordinate biological processes with the environment so that changes in conditions are anticipated and processes favorably phased. Despite the identification of numerous clock genes, knowledge of their molecular connectivity and influence on output programs remains limited. We recently showed LUX encodes a sequence-specific DNA-binding protein that directly regulates expression of the morning clock gene PRR9. We also showed that LUX interacts with the evening-phased proteins ELF3 and ELF4 to form a complex called the Evening Complex (EC). The EC binds the PIF4 and PIF5 promoters to control hypocotyl growth as a clock output. Here we provide evidence that LUX also recruits ELF3 to the PRR9 promoter. As with the PIF4 and PIF5 promoters, both LUX and its close homolog NOX are required for recruitment. Hence the entire EC likely functions together as part of the core clock oscillator to optimize plant fitness.

LUX ARRHYTHMO encodes a nighttime repressor of circadian gene expression in the Arabidopsis core clock.

Circadian clocks provide an adaptive advantage by allowing organisms to anticipate daily and seasonal environmental changes [1, 2]. Eukaryotic oscillators rely on complex hierarchical networks composed of transcriptional and posttranslational regulatory circuits [3]. In Arabidopsis, current representations of the circadian clock consist of three or four interlocked transcriptional feedback loops [3, 4]. Although molecular components contributing to different domains of these circuits have been described, how the loops are connected at the molecular level is not fully understood. Genetic screens previously identified LUX ARRHYTHMO (LUX) [5], also known as PHYTOCLOCK1 (PCL1) [6], an evening-expressed putative transcription factor essential for circadian rhythmicity. We determined the in vitro DNA-binding specificity for LUX by using universal protein binding microarrays; we then demonstrated that LUX directly regulates the expression of PSEUDO RESPONSE REGULATOR9 (PRR9), a major component of the morning transcriptional feedback circuit, through association with the newly discovered DNA binding site. We also show that LUX binds to its own promoter, defining a new negative autoregulatory feedback loop within the core clock. These novel connections between the archetypal loops of the Arabidopsis clock represent a significant advance toward defining the molecular dynamics underlying the circadian network in plants and provide the first mechanistic insight into the molecular function of the previously orphan clock factor LUX.

The More We Know, the Less We Understand?: Complexity of MAP Kinase Signaling.

Mitogen activated protein kinases (MAPKs) are prevalent signal transduction proteins in eukaryotes, and play multiple and important roles by responding to a variety of stimuli. Numerous papers provided evidence for extensive use of these modules in plants, and some recently emerging data might seem difficult to reconcile with previously reported studies. Here, we illustrate the difficulties and current challenges of studying plant MAPKs by discussing published studies on pathways comprising MEKK1, MKK1 and MPK4.

Staphylococcus aureus RNAIII coordinately represses the synthesis of virulence factors and the transcription regulator Rot by an antisense mechanism.

RNAIII is the intracellular effector of the quorum-sensing system in Staphylococcus aureus. It is one of the largest regulatory RNAs (514 nucleotides long) that are known to control the expression of a large number of virulence genes. Here, we show that the 3' domain of RNAIII coordinately represses at the post-transcriptional level, the expression of mRNAs that encode a class of virulence factors that act early in the infection process. We demonstrate that the 3' domain acts primarily as an antisense RNA and rapidly anneals to these mRNAs, forming long RNA duplexes. The interaction between RNAIII and the mRNAs results in repression of translation initiation and triggers endoribonuclease III hydrolysis. These processes are followed by rapid depletion of the mRNA pool. In addition, we show that RNAIII and its 3' domain mediate translational repression of rot mRNA through a limited number of base pairings involving two loop-loop interactions. Since Rot is a transcriptional regulatory protein, we proposed that RNAIII indirectly acts on many downstream genes, resulting in the activation of the synthesis of several exoproteins. These data emphasize the multitude of regulatory steps affected by RNAIII and its 3' domain in establishing a network of S. aureus virulence factors.

The Arabidopsis MAP kinase kinase MKK1 participates in defence responses to the bacterial elicitor flagellin.

Plants sense pathogens through both pathogen-associated molecular patterns and recognition of race-specific virulence factors, which induce basal defence or an accelerated defence (often manifest in the form of local cell death), respectively. A mitogen-activated protein kinase (MAPK) module in Arabidopsis was previously proposed to signal from perception of the bacterial elicitor flagellin to the activation of basal defence-related genes. Here, we present evidence for a parallel MAPK-signalling pathway involved in the response to flg22, a peptide corresponding to the most conserved domain of flagellin. The endogenous Arabidopsis MAP kinase kinase MKK1 is activated in cells treated with flg22, phosphorylates the MAPK MPK4 in vitro, and activates it in vivo in protoplasts. In mkk1 mutant plants, the activation by flg22 of MPK4 and two other flg22-induced MAPKs (MPK3 and MPK6) is impaired. In the mkk1 mutant, a battery of both flg22-induced and flg22-repressed genes show altered expression, indicating that MKK1 negatively regulates the activity of flagellin-responsive genes. Intriguingly, in contrast to the mpk4 mutant, mkk1 shows no morphological anomalies and is compromised in resistance to both virulent and avirulent Pseudomonas syringae strains. Thus, the MKK1 signalling pathway modulates the expression of genes responding to elicitors and plays an important role in pathogen defence.

The Agrobacterium oncogene AB-6b causes a graft-transmissible enation syndrome in tobacco.

Agrobacterium 6b oncogenes induce tumours and modify plant growth in various ways. Here we show that the AB-6b gene from strain AB4 placed under 2x35S promoter control (2x35S-AB-6b) induces a complex enation syndrome in transgenic Nicotiana tabacum plants, that also occurs in a few rare cases of genetic enations. In Arabidopsis thaliana, 2x35S-AB-6b induced radially symmetrical tubes on the abaxial side of the leaves, which must therefore be considered as the Arabidopsis equivalents of enations on other plant species. Tobacco and Arabidopsis 2x35S-AB-6b leaves contained small, supernumerary densely packed cells between the spongy mesophyll and the abaxial epidermis, close to vascular strands arising at an early stage of leaf development. On tobacco, the 2x35S-AB-6b enation syndrome could be transmitted across graft junctions to growing tissues of untransformed plants, both acropetally and basipetally. We propose that the AB-6b gene encodes the synthesis of one or more enation factor(s) that are transported by the phloem and modify the growth of developing tissues.

New plant growth-modifying properties of the Agrobacterium T-6b oncogene revealed by the use of a dexamethasone-inducible promoter.

Agrobacterium 6b oncogenes induce tumours on Nicotiana glauca and enations and associated modifications in transgenic N. tabacum plants. 2x35S-AB-6b tobacco rootstocks produced a graft-transmissible factor that induced enations in wild-type scions; the nature of this enation factor remains to be identified. Here, we report on the properties of tobacco plants carrying a dexamethasone-inducible T-6b gene (dex-T-6b). Induction with dex led to complex growth modifications, many of which have not been reported previously. Modifications were only found in growing tissues; mature tissues remained unaffected. Growth could be either stimulated or inhibited. Dex induction of young plants led to morphogenetic gradients that included enations, tubular leaves and fragmented leaf primordia. Root elongation was increased or slowed down, while radial root growth was strongly enhanced. Additional cell divisions were found in the root pericycle and vasculature. Enation factor import from mature tissues did not have the same effects on growing tissues as local T-6b synthesis: normal scions grafted on induced dex-T-6b rootstocks formed enations, whereas local dex-T-6b induction at the shoot apex led to numerous dark-green spots on the abaxial side of the leaves. In leaf patch assays, the 23-kDa T-6b protein was found to move through leaves and to enter the vascular system. This and the fact that rootstocks of spontaneous tobacco enation mutants did not modify wild-type scions contrary to 6b plants indicate that the 6b protein might be the enation factor.