Model for Alu retrotransposition. Alu RNA competes with 7SL RNA for SRP9/14 binding and RNP formation. It appears that at least one SRP9/14 heterdimer is necessary for Alu mobilization, although the binding of two heterodimers provides more efficient mobilization. Alu RNPs, once formed, can dock on ribosomes. As L1 mRNA is translated, the poly(A) tail of an SRP9/14-bound Alu competes for nascent L1 ORF2 reverse transcriptase (Sinnett et al. 1991; Boeke 1997; Dewannieux et al. 2003; Mills et al. 2007). Finally, a new Alu sequence is inserted into the genome by target-primed reverse transcription (Luan et al. 1993). Modern Alu RNAs have evolved weaker SRP9/14 binding affinities, perhaps to disengage from SRP9/14 more readily during reverse transcription.
