Role of poly(A) tail length in Alu retrotransposition

Alu are mobile noncoding Short INterspersed Elements (SINEs) present at a million copies in the human genome. Using marked Alu sequences in an ex vivo assay, we previously showed that they are mobilized through diversion of the LINE (Long INterspersed Elements) retrotransposition machinery, with the poly(A) tail of the Alu being required for their mobility. Here we show that other homopolymeric tracts cannot functionally replace the Alu poly(A) tail, and that the Alu transposition rate varies over a two-log range depending on the poly(A) tail length. Variation is according to a sigmoid-shaped curve with a lag observed for tails shorter than 15 nt and a plateau reached for tails longer than 50 nt, consistent with the binding of a limited number of a protein component requiring multiple contacts for a productive interaction with the poly(A) stretch. This analysis indicates that most of the naturally occurring genomic Alu, owing to their pA tail length, should be poor substrates for the LINE machinery, a feature possibly "selected" for the host sake.