Résumé
A large proportion of the plant LTR (Long Terminal Repeat) retrotransposons are partly or completely unable to synthesize their own machinery for transposition. However, most of these inactive or non-autonomous elements are likely able to retrotranspose, based on their insertional polymorphism. Therefore, they must be parasitic on one or more active partners. Here, we describe the parasitism of the chimeric
BARE-2 element on the active
BARE-1 (Barley RetroElement-2 and -1 respectively). These two elements are present in the Triticeae and related species, and are together polymorphic among closely related accessions.
BARE-2 elements are unable to synthesize their own GAG protein, and harbor a specific ATG deletion in the
gag ORF. However,
BARE-2 sequences are conserved with
BARE-1 in the
PBS (Primer Binding Site),
PSI (Packaging SIgnal) and
DIS (DImerization Signal) domains. As these motifs have been shown to allow parasitism among the lentiviruses, we conclude that
BARE-2 is probably a partial parasite of the
BARE-1 element because the machinery of the latter can complement the defective GAG of the former. This example emphasizes that we must characterize the parasitic network of LTR retrotransposons and its implication for integration of autonomous, inactive, and non-autonomous elements in order to understand current and past host genome evolution.