Polyribosome-Dependent Clustering of Membrane-Anchored RNA Degradosomes To Form Sites of mRNA Degradation in Escherichia coli

Here, we show that RNase E, RhlB, and PNPase act together as components of the multienzyme RNA degradosome in polyribosome-dependent clustering to form puncta on the inner cytoplasmic membrane. Our results support the hypothesis that RNA degradosome puncta are sites of mRNA degradation.

THE DEAD-BOX PROTEIN CSHA FROM Staphylococcus aureus Mu 50 EXHIBITS RIBONUCLEASE ACTIVITY

DEAD-box proteins play important roles in many RNA processes ranging from RNA synthesis to RNA decay. Furthermore, it has been reported that some bacterial DEAD-box proteins known to be components of the RNA degradosome do not cleave RNA substrates directly. However, the role of DEAD-box proteins in RNA degradation is poorly understood. The present study demonstrated that the DEAD-box protein CshA from the vancomycin-resistant Staphylococcus aureus strain Mu50 possesses RNA degradation activity, ribonuclease activity. Despite having RNA-dependent ATPase activity, CshA did not exhibit RNA helicase activity in vitro. Instead, CshA catalyzed the degradation of single-stranded RNAs of various duplex RNA substrates to form blunt-end RNA products. Thus, we suggest that the ribonuclease activity of the DEAD-box protein CshA may contribute to RNA remodeling in the bacterial RNA degradosome. To our knowledge, this study is the first to report that a DEAD-box protein from a pathogenic bacterium is implicated in multiple ATP-independent activity on RNA, such as degradation.

Acetylation regulates the oligomerization state and activity of RNase J, the major ribonuclease of Helicobacter pylori

In the pathogenic bacterium Helicobacter pylori, post-transcriptional regulation is dominated by the activity of a protein complex, known as the RNA degradosome, composed of the essential ribonuclease RNase J and the DEAD-box RNA helicase RhpA. Here, we describe post-translational modifications of this protein complex that affect its activity. Cell-extracted RNase J is acetylated on multiple residues, one of which, K649, impacts strongly on RNase J oligomerization, which in turn influences recruitment into the degradosome and ribonuclease activity. Corroborating the link between oligomerization and activity, mutations targeting K649 and other residues affect the dimerization and in vitro activity of RNase J. Our crystal structure of RNase J reveals loops that gate access to the active site and rationalizes how oligomerization state influences activity. We show that the acetylated residues of RNase J are important for H. pylori morphology, highlighting that the modifications affect the cellular function of RNase J. We propose acetylation as a regulatory level controlling the activity of RNase J and the H. pylori RNA degradosome.

RNase E endonuclease activity and its inhibition by pseudoridine

The conserved endoribonuclease RNase E dominates the dynamic landscape of RNA metabolism and underpins control mediated by small regulatory RNAs in diverse bacterial species. We explored the enzyme's hydrolytic mechanism, allosteric activation, and interplay with partner proteins in the multi-component RNA degradosome assembly. RNase E cleaves single-stranded RNA with preference to attack the phosphate located at the 5ʹ nucleotide preceding uracil, and we corroborate key interactions that select that base. Unexpectedly, RNase E activity is impeded strongly when the recognised uracil is isomerised to 5-ribosyluracil (pseudouridine), from which we infer the detailed geometry of the hydrolytic attack process. Kinetics analyses support models for recognition of secondary structure in substrates by RNase E and for allosteric auto-regulation. The catalytic power of the enzyme is boosted when it is assembled into the multi-enzyme RNA degradosome, most likely as a consequence of substrate channeling. Our results rationalize the origins of substrate preferences of RNase E and illuminate its catalytic mechanism, supporting the roles of allosteric domain closure and cooperation with other components of the RNA degradosome complex.

The association between Hfq and RNase E in long-term nitrogen starved Escherichia coli

The regulation of bacterial gene expression is underpinned by the synthesis and degradation of mRNA. In Escherichia coli, RNase E is the central enzyme involved in RNA degradation and serves as a scaffold for the assembly of the multiprotein complex known as the RNA degradosome. The activity of RNase E against specific mRNAs can also be regulated by the action of small RNAs (sRNA). The ubiquitous bacterial chaperone Hfq bound to sRNAs interacts with the RNA degradosome for the sRNA guided degradation of target mRNAs. The association between RNase E and Hfq has never been observed in live bacteria. We now show that in long-term nitrogen starved E. coli, both RNase E and Hfq co-localise in a single, large focus. This subcellular assembly, which we refer to as the H-body, also includes components of the RNA degradosome, namely, the helicase RhlB and the exoribonuclease polynucleotide phosphorylase. We further show that H-bodies are important for E. coli to optimally survive sustained nitrogen starvation. Collectively, the properties and features of the H-body suggests that it represents a hitherto unreported example of subcellular compartmentalisation of a process(s) associated with RNA management in stressed bacteria.

The RNase J-Based RNA Degradosome Is Compartmentalized in the Gastric Pathogen Helicobacter pylori

ABSTRACT Posttranscriptional regulation is a major level of gene expression control in any cell. In bacteria, multiprotein machines called RNA degradosomes are central for RNA processing and degradation, and some were reported to be compartmentalized inside these organelleless cells. The minimal RNA degradosome of the important gastric pathogen Helicobacter pylori is composed of the essential ribonuclease RNase J and RhpA, its sole DEAD box RNA helicase, and plays a major role in the regulation of mRNA decay and adaptation to gastric colonization. Here, the subcellular localization of the H. pylori RNA degradosome was investigated using cellular fractionation and both confocal and superresolution microscopy. We established that RNase J and RhpA are peripheral inner membrane proteins and that this association was mediated neither by ribosomes nor by RNA nor by the RNase Y membrane protein. In live H. pylori cells, we observed that fluorescent RNase J and RhpA protein fusions assemble into nonpolar foci. We identified factors that regulate the formation of these foci without affecting the degradosome membrane association. Flotillin, a bacterial membrane scaffolding protein, and free RNA promote focus formation in H. pylori. Finally, RNase J-GFP (RNase J-green fluorescent protein) molecules and foci in cells were quantified by three-dimensional (3D) single-molecule fluorescence localization microscopy. The number and size of the RNase J foci were found to be scaled with growth phase and cell volume as previously reported for eukaryotic ribonucleoprotein granules. In conclusion, we propose that membrane compartmentalization and the regulated clustering of RNase J-based degradosome hubs represent important levels of control of their activity and specificity. IMPORTANCE Helicobacter pylori is a bacterial pathogen that chronically colonizes the stomach of half of the human population worldwide. Infection by H. pylori can lead to the development of gastric pathologies such as ulcers and adenocarcinoma, which causes up to 800,000 deaths in the world each year. Persistent colonization by H. pylori relies on regulation of the expression of adaptation-related genes. One major level of such control is posttranscriptional regulation, which, in H. pylori, largely relies on a multiprotein molecular machine, an RNA degradosome, that we previously discovered. In this study, we established that the two protein partners of this machine are associated with the membrane of H. pylori. Using cutting-edge microscopy, we showed that these complexes assemble into hubs whose formation is regulated by free RNA and scaled with bacterial size and growth phase. Organelleless cellular compartmentalization of molecular machines into hubs emerges as an important regulatory level in bacteria.

The RNase J-based RNA degradosome is compartmentalized in the gastric pathogen Helicobacter pylori

Post-transcriptional regulation is a major level of gene expression control in any cell. In bacteria, multiprotein machines called RNA degradosomes are central for RNA processing and degradation and some were reported to be compartmentalized inside these organelle-less cells. The minimal RNA degradosome of the important gastric pathogen Helicobacter pylori is composed of the essential ribonuclease RNase J and RhpA, its sole DEAD-box RNA helicase, and plays a major role in the regulation of mRNA decay and adaptation to gastric colonization. Here, the subcellular localization of the H. pylori RNA degradosome was investigated using cellular fractionation and both confocal and super-resolution microscopy. We established that RNase J and RhpA are peripheral inner membrane proteins and that this association was mediated neither by ribosomes, by RNA nor by the RNase Y membrane protein. In live H. pylori cells, we observed that fluorescent RNase J and RhpA protein fusions assemble into non-polar foci. We identified factors that regulate the formation of these foci without affecting the degradosome membrane association. Flotillin, a bacterial membrane scaffolding protein, and free RNA promote foci formation in H. pylori. Finally, RNase J-GFP molecules and foci in cells were quantified by 3D-single-molecule fluorescence localization microscopy. The number and size of the RNase J foci were found to be scaled with growth phase and cell volume as was previously reported for eukaryotic ribonucleoprotein granules. In conclusion, we propose that membrane compartmentalization and the regulated clustering of RNase J-based degradosome hubs represent important levels of control of their activity and specificity.ImportanceHelicobacter pylori is a bacterial pathogen that chronically colonizes the stomach of half of the human population worldwide. Infection by H. pylori can lead to the development of gastric pathologies such as ulcers and adenocarcinoma, that causes up to 800.000 deaths in the world each year. Persistent colonization by H. pylori relies on regulation of the expression of adaptation-related genes. One major level of such control is post-transcriptional regulation that, in H. pylori, largely relies on a multi-protein molecular machine, an RNA-degradosome, that we previously discovered. In this study, we established that the two protein partners of this machine are associated to the membrane of H. pylori. Using cutting-edge microscopy, we showed that these complexes assemble into hubs whose formation is regulated by free RNA and scaled with bacterial size and growth phase. Cellular compartmentalization of molecular machines into hubs emerges as an important regulatory level in the organelle-less bacteria.