Alu RNA regulates the cellular pool of active ribosomes by targeted delivery of SRP9/14 to 40S subunits

Ivanova, Elena; Berger, Audrey; Scherrer, Anne; Alkalaeva, Elena; Strub, Katharina

Alu elements are derived from the 7SL RNA gene (5) , which encodes the RNA moiety of the signal recognition particle (SRP), and following retrotransposition Alu elements now make up over 10% of the human genome mass. The modern Alu element is composed of two similar, but not identical monomers named the left arm and the right arm ( Figure 1A ). Based on mutations at diagnostic positions, Alu elements are divided into three families of different evolutionary ages (8) . The most ancient family is named AluJ followed by AluS and AluY. Alu elements can be part of a polymerase II transcription unit and are therefore present in pre-mRNAs and mRNAs. These embedded Alu sequences can influence gene expression in many ways for example by providing alternative splice sites as well as target sites of miRNAs and editing enzymes (6) . Alu elements can also be transcribed by RNA polymerase III into the noncoding dimeric Alu RNA (7) , and under normal physiological conditions their expression is thought to be repressed by various cis-and trans-acting factors (8) most likely to keep retrotransposition events at low levels. However, their abundance is increased about 10-fold and higher in response to heat shock and viral infection (6) . Relatively little is known about cellular functions of Alu RNA. It has been shown to inhibit pol. II-dependent transcription, and its inhibitory activity was assigned to the right arm (9) .