Project description:Defining the in vivo folding pathway of cellular RNAs is essential to understand how they reach their final native conformation. We here introduce a novel method, named Structural Probing of Elongating Transcripts (SPET-seq), that permits single-base resolution analysis of transcription intermediates’ secondary structures on a transcriptome-wide scale, enabling base-resolution analysis of the RNA folding events. Our results suggest that cotranscriptional RNA folding in vivo is a mixture of cooperative folding events, in which local RNA secondary structure elements are formed as they get transcribed, and non-cooperative events, in which 5΄-halves of long-range helices get sequestered into transient non-native interactions until their 3΄ counterparts have been transcribed. Together our work provides the first transcriptome-scale overview of RNA cotranscriptional folding in a living organism.
Project description:Transcription elongation rates influence RNA processing, but sequence-specific regulation is poorly understood. We addressed this in vivo, analyzing RNAPI in S. cerevisiae. Mapping RNAPI by Miller chromatin spreads or UV crosslinking revealed 5′ enrichment and strikingly uneven local polymerase occupancy along the rDNA, indicating substantial variation in transcription speed. Two features of the nascent transcript correlated with RNAPI distribution: folding energy and GC content in the transcription bubble. In vitro experiments confirmed that strong RNA structures close to the polymerase promote forward translocation and limit backtracking, whereas high GC in the transcription bubble slows elongation. A mathematical model for RNAPI elongation confirmed the importance of nascent RNA folding in transcription. RNAPI from S. pombe was similarly sensitive to transcript folding, as were S. cerevisiae RNAPII and RNAPIII. For RNAPII, unstructured RNA, which favors slowed elongation, was associated with faster cotranscriptional splicing and proximal splice site use, indicating regulatory significance for transcript folding.
Project description:The yeast Hsp70 chaperone Ssb interacts with ribosomes and nascent chains to co-translationally assist protein folding. Here, we present a proteome-wide analysis of Hsp70 function during translation, based on in vivo selective ribosome profiling, that reveals mechanistic principles coordinating translation with chaperone-assisted protein folding. Ssb binds most cytosolic, nuclear, and mitochondrial proteins and a subset of ER proteins, supporting its general chaperone function. Position-resolved analysis of Ssb engagement reveals compartment- and protein-specific nascent chain binding profiles that are coordinated by emergence of positively charged peptide stretches enriched in aromatic amino acids. Ssbs’ function is temporally coordinated by RAC but independent from NAC. Analysis of ribosome footprint densities along orfs reveals that ribosomes translate faster at times of Ssb binding. This is coordinated by biases in mRNA secondary structure, and codon usage as well as the action of Ssb, suggesting chaperones may allow higher protein synthesis rates by actively coordinating protein synthesis with co-translational folding.
Project description:Guanine (G)-rich nucleic acids can fold into G-quadruplex (G4) structures under permissive conditions. Although many RNAs contain sequences that fold into RNA G4s (rG4s) in vitro, their folding and functions in vivo are not well understood. Here, we showed that the folding of putative rG4s in human cells into rG4 structures was dynamically induced by stress. By using high-throughput dimethylsulfate probing, we identified hundreds of endogenous stress-induced rG4s. Our results demonstrated that stress-induced rG4s were enriched in mRNA 3′-untranslated regions and enhanced mRNA stability. Furthermore, stress-induced rG4 folding was readily reversible upon stress removal. In summary, our study revealed the dynamic regulation of rG4 folding in living human cells and suggests suggested that widespread rG4 motifs may have a global regulatory impact on mRNA stability and cellular stress response.
Project description:To determine the prevalence of cotranscriptional splicing in Drosophila, we sequenced nascent RNA transcripts from Drosophila S2 cells as well as from Drosophila heads. 87% of introns assayed manifest more than 50% cotranscriptional splicing. The remaining 13% are cotranscriptionally spliced poorly, or slowly, with ~3% being almost completely retained in nascent pre-mRNA. Although individual introns showed slight but statistically significant differences in splicing efficiency, similar global levels of splicing were seen from both sources. Importantly, introns with low cotranscriptional splicing efficiencies are present in the same primary transcript with efficiently spliced introns, indicating that splicing is intron-specific. The analysis also indicates that cotranscriptional splicing is less efficient for first introns, longer introns and introns annotated as alternative. FinallyFinally, S2 cells expressing the slow RpII215C4 mutant manifest substantially less intron retention than wild-type S2 cells. Examination of Total pA and Nascent RNA from 2 different cell populations and isolated fly heads.