Retrotransposon BARE-1: expression of encoded proteins and formation of virus-like particles in barley cells

Retrotransposons are ubiquitous and major components of plant genomes, and are characteristically retroviral-like in their genomic structure and in th e major proteins encoded. Nevertheless, few have been directly demonstrated to be transcribed or reverse transcribed. The BARE-1 retrotransposon famil y of barley (Hordeum vulgare) is highly prevalent, actively transcribed, an d contains well conserved functional regions. Insertion sites for BARE-1 ar e highly polymorphic in the barley genome. Here we show that BARE-1 is tran slated and the capsid protein (GAG) and integrase (IN) components of the pr edicted polyprotein are processed into polypeptides of expected size. Some of the GAG sediments as virus-like particles together with IN and with BARE -1 cDNA. Reverse transcriptase activity is also present in gradient fractio ns containing BARE-1 translation products. Virus-like particles have also b een visualized in fractions containing BARE-1 components. Thus BARE-1 compo nents necessary for carrying out the life cycle of an active retrotransposo n appear to be present in vivo, and to assemble. This would suggest that po st-translational mechanisms may be at work to prevent rapid genome inflatio n through unrestricted integration.