Project description:Ribosome profiling has emerged as a powerful method to assess global gene translation, but methodological and analytical challenges often lead to inconsistencies across labs and model organisms. A critical issue in ribosome profiling is nuclease treatment of ribosome-mRNA complexes, as it is important to ensure both stability of ribosomal particles and complete conversion of polysomes to monosomes. We performed comparative ribosome profiling in yeast and mice with various ribonucleases including I, A, S7 and T1, characterized their cutting preferences, trinucleotide periodicity patterns, and coverage similarities across coding sequences, and showed that they yield comparable estimations of gene expression when ribosome integrity is not compromised. However, ribosome coverage patterns of individual transcripts had little in common between the ribonucleases. We further examined their potency at converting polysomes to monosomes across other commonly used model organisms, including bacteria, nematodes and fruit flies. In some cases, ribonuclease treatment completely degraded ribosome populations. Ribonuclease T1 was the only enzyme that preserved ribosomal integrity while thoroughly converting polysomes to monosomes in all examined species. This study provides a guide for ribonuclease selection in ribosome profiling experiments across most common model systems
Project description:Dysregulated protein synthesis is a major underlying cause of many neurodevelopmental diseases such as Fragile X Syndrome. A very robust technique is required to capture subtle but biologically significant differences in neurological disorders. Ribosome profiling, which is based on deep sequencing of mRNA fragments protected from ribonuclease digestion by ribosomes, is a powerful tool to study translational control. However, it has been mainly applied to rapidly dividing cells where translation is robust and where large amounts of starting material are readily available. The application of ribosome profiling to low-input brain tissue where translation is modest and where gene expression changes between genotypes are expected to be small has not been carefully evaluated. Using hippocampal tissue from wide type and fragile X mental retardation 1 (Fmr1) knockout mice, we show that variable RNase digestion can lead to significant sample batch effects. We also establish GC content and ribosome footprint length as quality control metrics for ribonuclease digestion. We performed ribonuclease titration experiments for low-input samples to identify optimal conditions for this critical step that is often improperly conducted. Our data reveal that optimal RNase digestion is essential to ensure high quality and reproducibility of ribosome profiling especially for low-input brain tissue.
Project description:The fidelity of start codon recognition by ribosomes is paramount during protein synthesis. The textbook knowledge of eukaryotic translation initiation depicts 5’→3’ unidirectional migration of the pre-initiation complex (PIC) along the 5’UTR. In probing translation initiation from ultra-short 5’UTR, we report that an AUG triplet near the 5’ end can be selected via PIC backsliding. The bi-directional ribosome scanning is supported by competitive selection of closely spaced AUG codons and recognition of two initiation sites flanking an internal ribosome entry site. Transcriptome-wide PIC profiling reveals footprints with an oscillation pattern near the 5’ end and start codons. Depleting the RNA helicase eIF4A leads to reduced PIC oscillations and impaired selection of 5’ end start codons. Enhancing the ATPase activity of eIF4A promotes nonlinear PIC scanning and stimulates upstream translation initiation. The helicase-mediated PIC conformational switch may provide an operational mechanism that unifies ribosome recruitment, scanning, and start codon selection.
Project description:Our study found that insertion of commonly used selection cassettes in yeast can drive aberrant transcription events and disrupt expression of neighboring genes. We performed ribosome profiling and mRNA-seq on yeast strains where ORFs had been replaced with common selection cassettes and observed aberrant transcripts. Aberrant transcripts positioned near adjacent genomic loci repressed protein synthesis of those genes by transcription interference (ablating normal length transcript production) coincident with translation of competitive uORFs on the long transcript which prevented translation of the main ORF seen by ribosome profiling.
Project description:Ribosome pauses are associated with diverse co-translational events and determine the fate of mRNAs and proteins. Thus the identification of the precise pause sites across transcriptome is a key, however, the landscape in bacterial has remained ambiguous. Here, we harnessed the multiple ribosome profiling strategies (standard, high-salt-wash, and disome) to survey the robust ribosome pause sites in E. coli. The found pause sites showed the correspondence with biochemical validation by integrated nascent chain profiling (iNP), which detects polypeptidyl-tRNA, an elongation intermediate. Among the list, ribosome pause at Asn586 of ycbZ was ensured by biochemical reporter assay, tRNA-seq, and cryo-electron microscopy. Our results provide a useful resource of ribosome stalling sites in bacteria.