Long terminal repeat (LTR) retrotransposons occupy a special place among all mobile genetic element families. The structure of LTR retrotransposons that have three open reading frames is identical to DNA forms of retroviruses that are integrated into the host genome. Several lines of evidence suggest that LTR retrotransposons share a common ancestry with retroviruses and thus are highly relevant to understanding mechanisms of transposition. Drosophila melanogaster is an exceptionally convenient model for studying the mechanisms of retrotransposon movement because many such elements in its genome are transpositionally active. Moreover, two LTRretrotransposons of D. melanogaster, gypsy and ZAM, have been found to have infectious properties and have been classified as errantiviruses. Despite numerous studies focusing on retroviral integration process, there is still no clear understanding of integration specificity in a target site. Most LTR retrotransposons non‐specifically integrate into a target site. Site‐specificity of integration at vertebrate retroviruses is rather relative. At the same time, sequence‐specific integration is the exclusive property of errantiviruses and their derivatives with two open reading frames. The possible basis for the errantivirus integration specificity is discussed in the present review.

Background: Retrotransposons with long terminal repeats (LTR retrotransposons) are widespread in all groups of eukaryotes and are often both the cause of new mutations and the source of new sequences. Apart from their high activity in generative and differentiation-stage tissues, LTR retrotransposons also become more active in response to different stressors. The precise causes of LTR retrotransposons’ activation in response to stress, however, have not yet been thoroughly investigated. Methods: We used RT-PCR to investigate the transcriptional profile of LTR retrotransposons and piRNA clusters in response to oxidative and chronic heat stresses. We used Oxford Nanopore sequencing to investigate the genomic environment of new insertions of the retrotransposons. We used bioinformatics methods to find the stress-induced transcription factor binding sites in LTR retrotransposons. Results: We studied the transposition activity and transcription level of LTR retrotransposons in response to oxidative and chronic heat stress and assessed the contribution of various factors that can affect the increase in their expression under stress conditions: the state of the piRNA-interference system, the influence of the genomic environment on individual copies, and the presence of the stress-induced transcription factor binding sites in retrotransposon sequences. Conclusions: The main reason for the activation of LTR retrotransposons under stress conditions is the presence of transcription factor binding sites in their regulatory sequences, which are triggered in response to stress and are necessary for tissue regeneration processes. Stress-induced transposable element activation can function as a trigger mechanism, triggering multiple signal pathways and resulting in a polyvariant cell response.

Full classification of Drosophila melanogasterretrotransposons with long terminal repeats (LTR-retrotransposons) has been recomposed, and their evolutional analysis in sequenced genomes of different...

Drosophila melanogaster is one of the most extensively used genetic model organisms for studying LTR retrotransposons that are represented by various groups in its genome. However, the phenomenon of molecular domestication of LTR retrotransposons has been insufficiently studied in Drosophila, as well as in other invertebrates. The present work is devoted to studying the role of the domesticated gag gene, Gagr, in the Drosophila genome. The Gagr gene has been shown to be involved in the response to stress caused by exposure to ammonium persulfate, but not in the stress response to oligomycin A, zeomycin, and cadmium chloride. Ammonium persulfate tissue specifically activates the expression of Gagr in the tissues of the carcass, but not in the gut. We found that the Gagr gene promoter contains one binding motif for the transcription factor kayak, a component of the JNK signaling pathway, and two binding motifs for the transcription factor Stat92E, a component of the Jak-STAT signaling pathway. Remarkably, Gagr orthologs contain the second binding motif for Stat92E only in D. melanogaster, D. simulans and D. sechellia, whereas in D. yakuba and D. erecta, Gagr orthologs contain a single motif, and there are no binding sites for Stat92E in the promoters of Gagr orthologs in D. ananassae and in species outside the melanogaster group. The data obtained indicate the formation of the protective function of the Gagr gene during evolution.

The most important sources of new components of genomes are transposable elements, which can occupy more than half of the nucleotide sequence of the genome in higher eukaryotes. Among the mobile components of a genome, a special place is occupied by retroelements, which are similar to retroviruses in terms of their mechanisms of integration into a host genome. The process of positive selection of certain sequences of transposable elements and retroviruses in a host genome is commonly called molecular domestication. There are many examples of evolutionary adaptations of gag (retroviral capsid) sequences as new regulatory sequences of different genes in mammals, where domesticated gag genes take part in placenta functioning and embryogenesis, regulation of apoptosis, hematopoiesis, and metabolism. The only gag-related gene has been found in the Drosophila genome—Gagr. According to the large-scale transcriptomic and proteomic analysis data, the Gagr gene in D. melanogaster is a component of the protein complex involved in the stress response. In this work, we consider the evolutionary processes that led to the formation of a new function of the domesticated gag gene and its adaptation to participation in the stress response. We discuss the possible functional role of the Gagr as part of the complex with its partners in Drosophila, and the pathway of evolution of proteins of the complex in eukaryotes to determine the benefit of the domesticated retroelement gag gene.