RESUMEN
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.
RESUMEN
The barley BARE-1 is a transcribed, copia-like retroelement with well-conserved functional domains, an active promoter, and a copy number of at least 3 x 10(4). We examined its chromosomal localization by in situ hybridization. The long terminal repeat (LTR) probe displayed a uniform hybridization pattern over the whole of all chromosomes, excepting paracentromeric regions, telomeres, and nucleolar organizer (NOR) regions. The integrase probe showed a similar pattern. The 5'-untranslated leader (UTL) probe, expected to be the most rapidly evolving component, labeled chromosomes in a dispersed and non-uniform manner, concentrated in the distal regions, possibly indicating a targe site preference.
