Through alternative polyadenylation, human mRNAs acquire longer or shorter 3' untranslated regions, the latter typically associated with higher transcript stability and increased protein production. To understand the dynamics of polyadenylation site usage, we performed transcriptome-wide mapping of both binding sites of 3' end processing factors CPSF-160, CPSF-100, CPSF-73, CPSF-30, Fip1, CstF-64, CstF-64τ, CF I(m)25, CF I(m)59, and CF I(m)68 and 3' end processing sites in HEK293 cells. We found that although binding sites of these factors generally cluster around the poly(A) sites most frequently used in cleavage, CstF-64/CstF-64τ and CFI(m) proteins have much higher positional specificity compared to CPSF components. Knockdown of CF I(m)68 induced a systematic use of proximal polyadenylation sites, indicating that changes in relative abundance of a single 3' end processing factor can modulate the length of 3' untranslated regions across the transcriptome and suggesting a mechanism behind the previously observed increase in tumor cell invasiveness upon CF I(m)68 knockdown.

Eukaryotic cells contain several unconventional poly(A) polymerases in addition to the canonical enzymes responsible for the synthesis of poly(A) tails of nuclear messenger RNA precursors. The yeast protein Trf4p has been implicated in a quality control pathway that leads to the polyadenylation and subsequent exosome-mediated degradation of hypomethylated initiator tRNAMet (tRNAi Met). Here we show that Trf4p is the catalytic subunit of a new poly(A) polymerase complex that contains Air1p or Air2p as potential RNA-binding subunits, as well as the putative RNA helicase Mtr4p. Comparison of native tRNAi Met with its in vitro transcribed unmodified counterpart revealed that the unmodified RNA was preferentially polyadenylated by affinity-purified Trf4 complex from yeast, as well as by complexes reconstituted from recombinant components. These results and additional experiments with other tRNA substrates suggested that the Trf4 complex can discriminate between native tRNAs and molecules that are incorrectly folded. Moreover, the polyadenylation activity of the Trf4 complex stimulated the degradation of unmodified tRNAi Met by nuclear exosome fractions in vitro. Degradation was most efficient when coupled to the polyadenylation activity of the Trf4 complex, indicating that the poly(A) tails serve as signals for the recruitment of the exosome. This polyadenylation-mediated RNA surveillance resembles the role of polyadenylation in bacterial RNA turnover.

In mammals, polyadenylation of mRNA precursors (premRNAs) by poly(A) polymerase (PAP) depends on cleavage and polyadenylation specificity factor (CPSF). CPSF is a multisubunit complex that binds to the canonical AAUAAA hexamer and to U-rich upstream sequence elements on the pre-mRNA, thereby stimulating the otherwise weakly active and nonspecific polymerase to elongate efficiently RNAs containing a poly(A) signal. Based on sequence similarity to the Saccharomyces cerevisiae polyadenylation factor Fip1p, we have identified human Fip1 (hFip1) and found that the protein is an integral subunit of CPSF. hFip1 interacts with PAP and has an arginine-rich RNA-binding motif that preferentially binds to U-rich sequence elements on the pre-mRNA. Recombinant hFip1 is sufficient to stimulate the in vitro polyadenylation activity of PAP in a U-rich element-dependent manner. hFip1, CPSF160 and PAP form a ternary complex in vitro, suggesting that hFip1 and CPSF160 act together in poly(A) site recognition and in cooperative recruitment of PAP to the RNA. These results show that hFip1 significantly contributes to CPSF-mediated stimulation of PAP activity.

Alternative polyadenylation (APA) is a general mechanism of transcript diversification in mammals, which has been recently linked to proliferative states and cancer. Different 3′ untranslated region (3′ UTR) isoforms interact with different RNA-binding proteins (RBPs), which modify the stability, translation, and subcellular localization of the corresponding transcripts. Although the heterogeneity of pre-mRNA 3′ end processing has been established with high-throughput approaches, the mechanisms that underlie systematic changes in 3′ UTR lengths remain to be characterized. Through a uniform analysis of a large number of 3′ end sequencing data sets, we have uncovered 18 signals, six of which are novel, whose positioning with respect to pre-mRNA cleavage sites indicates a role in pre-mRNA 3′ end processing in both mouse and human. With 3′ end sequencing we have demonstrated that the heterogeneous ribonucleoprotein C (HNRNPC), which binds the poly(U) motif whose frequency also peaks in the vicinity of polyadenylation (poly(A)) sites, has a genome-wide effect on poly(A) site usage. HNRNPC-regulated 3′ UTRs are enriched in ELAV-like RBP 1 (ELAVL1) binding sites and include those of the CD47 gene, which participate in the recently discovered mechanism of 3′ UTR–dependent protein localization (UDPL). Our study thus establishes an up-to-date, high-confidence catalog of 3′ end processing sites and poly(A) signals, and it uncovers an important role of HNRNPC in regulating 3′ end processing. It further suggests that U-rich elements mediate interactions with multiple RBPs that regulate different stages in a transcript's life cycle.

RNA-specific nucleotidyl transferases (rNTrs) are a diverse family of template-independent polymerases that add ribonucleotides to the 39-ends of RNA molecules. All rNTrs share a related active-site architecture first described for DNA polymerase b and a catalytic mechanism conserved among DNA and RNA polymerases. The best known examples are the nuclear poly(A) polymerases involved in the 39-end processing of eukaryotic messenger RNA precursors and the ubiquitous CCA-adding enzymes that complete the 39-ends of tRNA molecules. In recent years, a growing number of new enzymes have been added to the list that now includes the ''noncanonical'' poly(A) polymerases involved in RNA quality control or in the readenylation of dormant messenger RNAs in the cytoplasm. Other members of the group are terminal uridylyl transferases adding single or multiple UMP residues in RNA-editing reactions or upon the maturation of small RNAs and poly(U) polymerases, the substrates of which are still not known. 29-59Oligo(A) synthetases differ from the other rNTrs by synthesizing oligonucleotides with 29-59-phosphodiester bonds de novo.