R2 elements are non-long terminal repeat (non-LTR) retrotransposons that insert site-specifically into the host organism's 28S ribosomal RNA (rRNA) genes. To date these molecular parasites have been identified in arthropods, vertebrates, echinoderms, flatworms and hydra. Once a 28S gene is inserted it is no longer capable of producing functional rRNA; however, the ribosomal DNA locus itself is typically comprised of hundreds of genes. The “ecological landscape” for propagation of R2 elements is thus complex: the R2 element must produce enough copies to maintain itself, but overinsertion by R2 is detrimental to the host, so the elements must also contend with host mechanisms that suppress R2 replication. Originally discovered in 1977, many aspects of the biology of R2 retrotransposons have been deduced over subsequent years using silkmoths and fruitflies as model systems. As a retrotransposon, the R2 element reproduces itself through an RNA intermediate: the R2 RNA, which possesses a single ORF for the R2 protein as well as cis-regulatory signals necessary for insertion into the host genome. The R2 RNA is initially co-transcribed with host ribosomal RNA by RNA polymerase I and is processed out of this primary transcript utilizing an encoded self-cleaving ribozyme to free the 5′ end. The R2 RNA interacts with the R2 protein during insertion into the host genome. The mechanism by which the R2 element inserts has been well documented for Bombyx mori and proceeds via an ordered sequence of DNA cleavage and synthesis. Here you can see a recent cryoEM structure of the R2 with 5'ORF from Bombyx mori (PDB code: 8IBY)

#molecularart ... #immolecular ... #retrotransposon ... #DNA ... #complex ... #bombyx ... #cryoem

Structure rendered with @proteinimaging and depicted with @corelphotopaint