Envelope-class retrovirus-like elements are widespread, transcribed and spliced, and insertionally polymorphic in plants

Retrotransposons and retroviruses share similar intracellular life cycles a nd major encoded proteins, but retrotransposons lack the envelope (env) cri tical for infectivity. Retrotransposons are ubiquitous and abundant in plan ts and active retroviruses are known in animals. Although a few em-containi ng retroelements, gypsy-like Athila, Cyclops, and Calypso and copia-like SI RE-1, have been identified in plants, the general presence and functionalit y of the domain remains unclear. We show here that env-class elements are p resent throughout the flowering plants and are widely transcribed. Within t he grasses, we show the transcription of the env domain itself for Bagy-2 a nd related retrotransposons, all members of the Athila group. Furthermore, Bagy-2 transcripts undergo splicing to generate a subgenomic env product as do those of retroviruses. Transcription and the polymorphism of their inse rtion sites in closely related barley cultivars suggests that at least some are propagationally active. The putative ENV polypeptides of Bagy-2 and ri ce Rigy-2 contain predicted leucine zipper and transmembrane domains typica l of retroviral ENVs. These findings raise the prospect of active retrovira l agents among the plants.