The 3' extremity of mRNA unveiled by TAIL-seq

Global investigation of the 3' extremity of mRNA, despite its importance in gene regulation, has not been feasible due to technical challenges associated with homopolymeric sequences and relative paucity of mRNA. By developing a new methodology, TAIL-seq, we find a number of unforeseen features at the 3' termini of mRNA. Our analyses reveal the transcriptome-wide distribution of poly(A) tail and estimate the median poly(A) length as 55-60 nt in mammalian cells, which is substantially shorter than generally conceived. Poly(A) length correlates with mRNA half-life but not with translation efficiency. Surprisingly, we find widespread uridylation and guanylation at the downstream of poly(A) tail. The U-tails are generally attached to short poly(A) tails (<25 nt) while the G-tails are found mainly on longer poly(A) tails (>40 nt), implicating their generic roles in mRNA stability control. In addition, TAIL-seq identifies, with a single nucleotide resolution, numerous nucleolytic events involved in microRNA processing and mRNA cleavage. Overall design: Single replicate without any special experimental treatment for each of mouse fibroblast cell line NIH3T3 and human cervical cell line HeLa.