Integration of retroposable elements in mammals: selection of target sites.

Genomic DNA fragments generated by the reverse transcription of cellular RNA are called retroposons. Because they are flanked by short repeats, mammalian retroposons are believed to integrate at staggered chromosomal breaks. Recently, a significant sequence pattern associated with the integration of Alu and ID repeats was identified (Jurka 1996). It is represented by the 5' TTAAAA consensus sequence around the 5' ends of flanking repeats of Alu, ID, as well as, of B1 and B2 retroposed elements as shown in this paper. This consensus is a potential target for enzymatic nicking which probably occurs in the complementary strand between 3' AA and the following 3' TTTT bases. The first four bases of the flanking repeats corresponding to the 3' TTTT consensus sequence show some sequence variations that may be affected by complementary base pairing between the A-rich RNA tails and the DNA target sequences prior to nicking and reverse transcription. We discuss potential evidence for such base pairing based on correlated variations in nucleotide composition of different tail and target regions.