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

Abstract

Summary 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.

A Contiguous 66-kb Barley DNA Sequence Provides Evidence for Reversible Genome Expansion

“…Dot blot data and combined restriction mapping and hybridization analyses of BAC (Bacterial Artificial Chromosome) clones indicated that, in barley and other Hordeum species, BARE-1 LTRs vastly outnumber (16 ‫ע‬ 2 ‫)ן‬ BARE-1 elements containing internal domains. (Jääskeläinen et al 1999;Vicient et al 1999). These data would be consistent with intraelement LTR recombination generating solo LTRs, at least of BARE-1.…”

“…It is transcribed in somatic tissues (Suoniemi et al 1996) and is translated, processed, and assembled into virus-like particles (Jääskeläinen et al 1999). Dot blot data and combined restriction mapping and hybridization analyses of BAC (Bacterial Artificial Chromosome) clones indicated that, in barley and other Hordeum species, BARE-1 LTRs vastly outnumber (16 ‫ע‬ 2 ‫)ן‬ BARE-1 elements containing internal domains.…”

Frequencies of Ty1-copia and Ty3-gypsy retroelements within the Triticeae EST databases

“…Previous studies have shown that some of these retroelements are also transcribed in different Triticeae species (Pearce et al 1997;Suoniemi et al 1996b) though no quantification of their abundance is available. Translation products of Ty1-copia-type retrotransposons have been recently detected immunologically in barley (Jääskeläinen et al 1999).…”

Retrotransposons and their Role in Plant—Genome Evolution

“…Reverse transcription follows transcription, and depends upon a (-)-strand initiation site (PBS), which is generally complementary to a tRNA in LTR retrotransposons, and a (+) strand initiation site (PPT), which is purinerich. These are highly conserved both for BARE-1 (82) and LARD elements (39) (36). Furthermore, BARE-1 GAG antibodies also detect a full-length translation product and its proteolytic processing products from barley and other cereals including rice (90).…”

“…Translation of a third encoded domain, for the envelope protein, has not yet been directly shown in barley for the Bagy-2 element, although the requisite RNA splicing takes place (89). The GAG and integrase products of BARE-1 appear able to form virus-like particles, which can be visualized from sucrose gradients, associated with reverse transcriptase activity (36).…”