Several classes of small noncoding RNAs are key players in cellular metabolism including mRNA decoding, RNA processing, and mRNA stability. Here we show that a tRNA Asp isodecoder, corresponding to a human tRNA-derived sequence, binds to an embedded Alu RNA element contained in the 3′ UTR of the human aspartyl-tRNA synthetase mRNA. This interaction between two well-known classes of RNA molecules, tRNA and Alu RNA, is driven by an unexpected structural motif and induces a global rearrangement of the 3′ UTR. Besides, this 3′ UTR contains two functional polyadenylation signals. We propose a model where the tRNA/Alu interaction would modulate the accessibility of the two alternative polyadenylation sites and regulate the stability of the mRNA. This unique regulation mechanism would link gene expression to RNA polymerase III transcription and may have implications in a primate-specific signal pathway.aminoacyl-tRNA synthetase | posttranscriptional regulation A lu elements are the most abundant repetitive element in primate genomes. They belong to the short interspersed element family and are classified into subfamilies according to their appearance during evolution. Typically, there are more than 1 million Alu elements in a primate genome, which represents 10% of the genome overall. Modern Alu elements are about 300 base-pairs (bp) long and adopt a conserved structure composed of two similar, but distinct, domains (left and right arms) joined by an A-rich linker and followed by a short polyadenylation [poly (A)] stretch. Moreover, Alu elements are transcribed by RNA polymerase III (pol III), and Alu RNA transcripts are present in the cytosol of primate cells (1).Alu sequences are considered to be a huge reservoir of potential regulatory elements, which may have been involved in primate evolution and possibly facilitated their divergence from other mammals (2). Indeed, insertion of Alu elements could provide previously undescribed regulatory motifs to neighboring genes by creating previously undescribed transcription enhancers or promoters (3, 4). Alu elements can also modulate protein expression in a posttranscriptional way by at least two different mechanisms: (i) they are transcribed as free Alu RNA by the RNA PolIII, assembled into Alu ribonucleo-protein particles and act as transregulatory factors (2, 5, 6); or (ii) their transcription as a part of a mRNA will define them as cis-regulatory elements (7). If the Alu sequence is localized within the mRNA open reading frame, it can either affect gene expression by sequence disruption or, alternatively, simply add to the protein amino acid sequence (8). Nevertheless, most of the time they are present in 5′ and 3′ UTRs (2, 9, 10).Several reports indicate that mRNAs containing Alu elements in their 3′ UTRs are associated with cell growth and differentiation (11-15). Indeed, their presence could affect the processing of mRNAs at multiple levels (16). Specifically, they have been shown to modulate alternative splicing (e.g., ref. 17), RNA editing (e.g., ref. 18), nuclear ...