Rapid nuclear deadenylation of mammalian messenger RNA

Poly(A) tails protect RNAs from degradation and deadenylation rates determine RNA stability. Deadenylation has mostly been investigated within the cytoplasm and the dynamics of poly(A) tail length after transcription are not well understood. Combining long-read sequencing with metabolic labeling and cell fractionations, we investigate deadenylation dynamics of newly synthesized poly(A) tails in vitro and in vivo. We report evidence for genome-wide synthesis of poly(A) tails longer than 200 nt which are enriched upon splicing inhibition. Metabolic labeling reveals rapid deadenylation of poly(A) tails within minutes after transcription. Fractionation experiments show that initial deadenylation is a nuclear process, and that different classes of transcripts, including long noncoding RNAs, have distinctive nuclear poly(A) tail lengths. Modelling deadenylation dynamics predicts that deadenylation in the nucleus is by an order of magnitude faster than in the cytoplasm. In summary, we suggest nuclear deadenylation as a novel regulatory layer which may determine stability and abundance before mRNAs reach the cytoplasm. Overall design: Full length poly(A) and mRNA sequencing (FLAM-seq) analysis of PlaB treated HeLa nuclei, pulldown experiments, SLAM-seq, and biochemical fractionation