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AbstractReverse transcriptases (RT), or RNA-dependent DNA polymerases, are unorthodox enzymes that originally added a new angle to the conventional view of the unidirectional flow of genetic information in the cell from DNA to RNA to protein. First discovered in vertebrate retroviruses, RTs were since re-discovered in most eukaryotes, bacteria, and archaea, spanning essentially all domains of life. For retroviruses, RTs provide the ability to copy the RNA genome into DNA for subsequent incorporation into the host genome, which is essential for their replication and survival. In cellular organisms, most RT sequences originate from retrotransposons, the type of self-replicating genetic elements that rely on reverse transcription to copy and paste their sequences into new genomic locations. Some retroelements, however, can undergo domestication, eventually becoming a valuable addition to the overall repertoire of cellular enzymes. They can be beneficial yet accessory, like the diversity-generating elements, or even essential, like the telomerase reverse transcriptases. Nowadays, ever-increasing numbers of domesticated RT-carrying genetic elements are being discovered. It may be argued that domesticated RTs and reverse transcription in general is more widespread in cellular organisms than previously thought, and that many important cellular functions, such as chromosome end maintenance, may evolve from an originally selfish process of converting RNA into DNA. 

Abstract Eukaryotic retroelements are generally divided into two classes: long terminal repeat (LTR) retrotransposons and non-LTR retrotransposons. A third class of eukaryotic retroelement, the Penelope-like elements (PLEs), has been well-characterized bioinformatically, but relatively little is known about the transposition mechanism of these elements. PLEs share some features with the R2 retrotransposon fromBombyx mori, which uses a target-primed reverse transcription (TPRT) mechanism, but their distinct phylogeny suggests PLEs may utilize a novel mechanism of mobilization. Using protein purified fromE. coli, we report unique in vitro properties of a PLE from the green anole (Anolis carolinensis), revealing mechanistic aspects not shared by other retrotransposons. We found that reverse transcription is initiated at two adjacent sites within the transposon RNA that is not homologous to the cleaved DNA, a feature that is reflected in the genomic “tail” signature shared between and unique to PLEs. Our results for the first active PLE in vitro provide a starting point for understanding PLE mobilization and biology. 

Abstract The conference “Transposable Elements at the Crossroads of Evolution, Health and Disease” was hosted by Keystone Symposia in Whistler, British Columbia, Canada, on September 3–6, 2023, and was organized by Kathleen Burns, Harmit Malik and Irina Arkhipova. The central theme of the meeting was the incredible diversity of ways in which transposable elements (TEs) interact with the host, from disrupting the existing genes and pathways to creating novel gene products and expression patterns, enhancing the repertoire of host functions, and ultimately driving host evolution. The meeting was organized into six plenary sessions and two afternoon workshops with a total of 50 invited and contributed talks, two poster sessions, and a career roundtable. The topics ranged from TE roles in normal and pathological processes to restricting and harnessing TE activity based on mechanistic insights gained from genetic, structural, and biochemical studies.