A transcriptome-wide atlas of RNP compositions reveals diverse classes of mRNAs and lncRNAs
Aim: To determine how different classes of transcript (e.g. lncRNAs and mRNAs) are defined in the cell. Approach: We determined the transcriptome-wide targets of key RNA packaging, maturation, export and turnover factors. We used the CRAC technique, whereby RNA:protein interactions are fixed by UV irradiation of yeast cultures, and RNA:protein complexes obtained via a stringent multi-step purification. A mild RNase treatment fragments the bound RNAs, which are then used as templates for RT-PCR, prior to sequencing. This approach enabled us to compare the maturation and turnover pathways of mRNAs and lncRNAs. Results: Our data reveal that mRNA and lncRNA maturation pathways diverge prior to nuclear export, and 3' end processing emerges as a key step in determining transcript fate. Our analyses also reveal when and where the tested proteins bind to mRNAs, and thus offer much insight into the dynamic assembly of mRNPs. Analyses of reads with non-genome-encoded A-tails enabled us to distinguish proteins bound to stable poly(A) tails on full-length mRNAs, and to short oligo(A)4-5 tails on nuclear surveillance intermediates. This lead to the identification of a novel class of promoter-proximal ncRNAs, that we suggest arise from early termination within protein-coding genes. Overall design: Identification of targets of RNA packaging, processing, export and turnover factors in wild-type cells; replicates included for some but not all samples; “BY” samples are negative controls, which use untagged strains