The ability to form intrastrand tetraplexes is an evolutionarily conserved feature of the 3' end of L1...

Mammalian genomes contain many thousands of members of a family of retrotransponsons known as L1 (or LINE-1) elements. These elements lack long terminal repeats (LTRs), and are thought to use a retroposition mechanism than differs from that of retroviruses and other LTR-containing retroelements. In order to define those regions of the L1 element that may be important for L1 retroposition, we examined the 3' untranslated regions (UTRs) of L1 elements from a diverse group of mammals. We show that while the 3' UTRs of L1 elements from different species share little if any sequence homology, they all contain a G-rich polypurine tract of variable length and sequence which can form one or more intrastrand tetraplexes. This conservation over the 100 Myr since the mammalian radiation suggests that either the G-rich motif itself or a conserved structure such as the tetraplex that can be formed by this motif is a significant structural feature of L1 elements and may play a role in their propagation.