Truncated forms of U2 snRNA (U2-tfs) are shunted toward a novel uridylylation pathway that differs from the degradation pathway for U1-tfs

During the biogenesis of U1 small nuclear ribonucleoprotein, a small population of U1 snRNA molecules acquires an extra methylation at the first transcribed nucleotide and a nucleolytic cleavage to remove the 3′ structured region including the Sm protein–binding site and stem-loop 4. These modifications occur before hypermethylation of the monomethylated 5′ cap, whereby producing truncated forms of U1 snRNA (U1-tfs) that are diverted from the...