Mixed tailing by TUT3 and TUT5 shields mRNA from rapid deadenylation [TAIL-Seq]

RNA tails play integral roles in the control of mRNA translation and decay. Guanylation of poly(A) tail was discovered recently, yet the enzymology and function remain obscure. Here we identify TUT3 (PAPD5) and TUT5 (PAPD7) (TUT3/5) as the enzymes responsible for mRNA guanylation. Purified TUT3/5 predominantly incorporate GTPs to generate a mixed poly(A) tail with intermittent non-adenosine residues. A single guanosine is sufficient to impede the deadenylation complex (CCR4-NOT) which trims the tail and exposes guanosine at the 3′ end. Consistently, the depletion of TUT3/5 leads to a decrease in mRNA half-­life and abundance in cells. Thus, TUT3/5 produce a mixed tail which shields the mRNA from rapid deadenylation. Our study unveils the role of mixed tailing, and expands the complexity of post-transcriptional gene regulation. HeLa cells and HFF cells were knocked down of control or TUT3/5 with two different siRNAs.