Spatial organization of single mRNPs at different stages of the gene expression pathway

mRNAs form ribonucleoprotein complexes (mRNPs) by association with proteins that are crucial for mRNA metabolism. While the mRNP proteome has been well characterized, little is known about mRNP organization. Using a single molecule approach, we show that mRNA conformation changes depending on its cellular localization and translational state. Compared to nuclear mRNPs and lncRNPs, association with ribosomes decompacts individual mRNAs, while their sequestration into stress-granules leads to increased compaction. Moreover, translating mRNAs rarely show co-localizing 5’ and 3’ ends, indicating that mRNAs are either not translated in a closed-loop configuration, or that mRNA circularization is transient, suggesting that a stable closed-loop conformation is not a universal state for all translating mRNAs. One Sentence Summary Single mRNA studies in cells show RNA compaction changes depending on translational state, but mRNAs are not translated in closed-loop conformation.