BARE, a Multiply Mobile Retrotransposon

Retrotransposons are ubiquitous and abundant transposable elements in eukaryotic genomes. They are responsible for much of the variation in monoploid genome size over evolutionary time and have direct or indirect effects on genes by both insertional mutagenesis and epigenetic mechanisms. Over 74% of the large (5.1 Gb) barley genome is comprised of long terminal repeat (LTR) retrotransposons. Retrotransposon copies may be passed to the next generation by one of two ways: replication of existing copies as part of the chromosome; synthesis and propagation of new copies by the combined activities of RNA polymerase II and reverse transcriptase by a “copy and paste” life cycle virtually identical to that of retroviruses, excepting (so far as has been directly demonstrated) the ability to readily move from organism to organism infectively. The retrotransposon life cycle within the cell involves both nuclear and cytoplasmic phases. Integrated copies are transcribed within the nucleus, but translation and virus-like particle assembly takes place in the cytoplasm. Following replication, the cytoplasmic, packaged cDNA copies need to be brought back into the nucleus for integration of new copies to occur. We are interested in analyzing the intracellular trafficking of retrotransposon components, using the BARE retrotransposon family of barley as the model system. The goal is to develop an understanding of the mechanisms controlling the propagation of retrotransposons and consequences for the genome and plant.