Uridylation occurs pervasively on mRNAs, yet its
mechanism and significance remain unknown. By
applying TAIL-seq, we identify TUT4 and TUT7
(TUT4/7), also known as ZCCHC11 and ZCCHC6,
respectively, as mRNA uridylation enzymes. Uridylation
readily occurs on deadenylated mRNAs in cells.
Consistently, purified TUT4/7 selectively recognize
and uridylate RNAs with short A-tails (less than
25 nt) in vitro. PABPC1 antagonizes uridylation of
polyadenylated mRNAs, contributing to the specificity
for short A-tails. In cells depleted of TUT4/7,
the vast majority of mRNAs lose the oligo-U-tails,
and their half-lives are extended. Suppression of
mRNA decay factors leads to the accumulation of
oligo-uridylated mRNAs. In line with this, microRNA
induces uridylation of its targets, and TUT4/7 are
required for enhanced decay of microRNA targets.
Our study explains the mechanism underlying
selective uridylation of deadenylated mRNAs and
demonstrates a fundamental role of oligo-U-tail as
a molecular mark for global mRNA decay.