Initiation of translation in bacteria by a structured eukaryotic IRES RNA.

The central dogma of gene expression (DNA to RNA to protein) is universal, but in different domains of life there are fundamental mechanistic differences within this pathway. For example, the canonical molecular signals used to initiate protein synthesis in bacteria and eukaryotes are mutually exclusive. However, the core structures and conformational dynamics of ribosomes that are responsible for the translation steps that take place after initiation are ancient and conserved across the domains of life. We wanted to explore whether an undiscovered RNA-based signal might be able to use these conserved features, bypassing mechanisms specific to each domain of life, and initiate protein synthesis in both bacteria and eukaryotes. Although structured internal ribosome entry site (IRES) RNAs can manipulate ribosomes to initiate translation in eukaryotic cells, an analogous RNA structure-based mechanism has not been observed in bacteria. Here we report our discovery that a eukaryotic viral IRES can initiate translation in live bacteria. We solved the crystal structure of this IRES bound to a bacterial ribosome to 3.8 Å resolution, revealing that despite differences between bacterial and eukaryotic ribosomes this IRES binds directly to both and occupies the space normally used by transfer RNAs. Initiation in both bacteria and eukaryotes depends on the structure of the IRES RNA, but in bacteria this RNA uses a different mechanism that includes a form of ribosome repositioning after initial recruitment. This IRES RNA bridges billions of years of evolutionary divergence and provides an example of an RNA structure-based translation initiation signal capable of operating in two domains of life.

Oxaliplatin disrupts nucleolar function through biophysical disintegration.

Platinum (Pt) compounds such as oxaliplatin are among the most commonly prescribed anti-cancer drugs. Despite their considerable clinical impact, the molecular basis of platinum cytotoxicity and cancer specificity remain unclear. Here we show that oxaliplatin, a backbone for the treatment of colorectal cancer, causes liquid-liquid demixing of nucleoli at clinically relevant concentrations. Our data suggest that this biophysical defect leads to cell-cycle arrest, shutdown of Pol I-mediated transcription, and ultimately cell death. We propose that instead of targeting a single molecule, oxaliplatin preferentially partitions into nucleoli, where it modifies nucleolar RNA and proteins. This mechanism provides a general approach for drugging the increasing number of cellular processes linked to biomolecular condensates.

Biophysical characterization of Chlamydia trachomatis CT584 supports its potential role as a type III secretion needle tip protein.

Chlamydia are obligate intracellular bacterial pathogens that cause a variety of diseases. Like many Gram-negative bacteria, they employ type III secretion systems (T3SS) for invasion, establishing and maintaining their unique intracellular niche, and possibly cellular exit. Computational structure prediction indicated that ORF CT584 is homologous to other T3SS needle tip proteins. Tip proteins have been shown to be localized to the extracellular end of the T3SS needle and play a key role in controlling secretion of effector proteins. We have previously demonstrated that T3SS needle tip proteins from different bacteria share many biophysical characteristics. To support the hypothesis that CT584 is a T3SS needle tip protein, biophysical properties of CT584 were explored as a function of pH and temperature, using spectroscopic techniques. Far-UV circular dichroism, Fourier transform infrared spectroscopy, UV absorbance spectroscopy, ANS extrinsic fluorescence, turbidity, right angle static light scattering, and analytical ultracentrifugation were all employed to monitor the secondary, tertiary, quaternary, and aggregation behavior of this protein. An empirical phase diagram approach is also employed to facilitate such comparisons. These analyses demonstrate that CT584 shares many biophysical characteristics with other T3SS needle tip proteins. These data support the hypothesis that CT584 is a member of the same functional family, although future biologic analyses are required.

Viral RNA structure-based strategies to manipulate translation.

Viruses must co-opt the cellular translation machinery to produce progeny virions. Eukaryotic viruses have evolved a variety of ways to manipulate the cellular translation apparatus, in many cases using elegant RNA-centred strategies. Viral RNAs can alter or control every phase of protein synthesis and have diverse targets, mechanisms and structures. In addition, as cells attempt to limit infection by downregulating translation, some of these viral RNAs enable the virus to overcome this response or even take advantage of it to promote viral translation over cellular translation. In this Review, we present important examples of viral RNA-based strategies to exploit the cellular translation machinery. We describe what is understood of the structures and mechanisms of diverse viral RNA elements that alter or regulate translation, the advantages that are conferred to the virus and some of the major unknowns that provide motivation for further exploration.

Translation initiation by the hepatitis C virus IRES requires eIF1A and ribosomal complex remodeling.

Internal ribosome entry sites (IRESs) are important RNA-based translation initiation signals, critical for infection by many pathogenic viruses. The hepatitis C virus (HCV) IRES is the prototype for the type 3 IRESs and is also invaluable for exploring principles of eukaryotic translation initiation, in general. Current mechanistic models for the type 3 IRESs are useful but they also present paradoxes, including how they can function both with and without eukaryotic initiation factor (eIF) 2. We discovered that eIF1A is necessary for efficient activity where it stabilizes tRNA binding and inspects the codon-anticodon interaction, especially important in the IRES' eIF2-independent mode. These data support a model in which the IRES binds preassembled translation preinitiation complexes and remodels them to generate eukaryotic initiation complexes with bacterial-like features. This model explains previous data, reconciles eIF2-dependent and -independent pathways, and illustrates how RNA structure-based control can respond to changing cellular conditions.