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 the major proteins encoded. Nevertheless, few have been directly demonstrated to be transcribed or reverse transcribed. The BARE-1 retrotransposon family of barley (Hordeum vulgare) is highly prevalent, actively transcribed, and contains well conserved functional regions. Insertion sites for BARE-1 are highly polymorphic in the barley genome. Here we show that BARE-1 is translated and the capsid protein (GAG) and integrase (IN) components of the predicted 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 fractions containing BARE-1 translation products. Virus-like particles have also been visualized in fractions containing BARE-1 components. Thus BARE-1 components necessary for carrying out the life cycle of an active retrotransposon appear to be present in vivo, and to assemble. This would suggest that post-translational mechanisms may be at work to prevent rapid genome inflation through unrestricted integration.