The Long Terminal Repeat promoter of Retroviruses contains stable and conserved G-quadruplexes.

Retroviruses infect almost all vertebrates, from humans to domestic and farm animals, from primates to wild animals, where they cause severe diseases, including immunodeficiencies, neurological disorders and cancer. Non-human retroviruses have also been recently associated to human diseases. To date no effective treatments are available, therefore finding retrovirus-specific therapeutic targets is becoming an impellent issue. G-quadruplexes are four-stranded nucleic acid structures that form in guanine-rich regions. Highly conserved G-quadruplexes located in the long terminal repeat (LTR) promoter of HIV-1 were shown to modulate the virus transcription machinery: moreover, the astonishingly high degree of conservation of G-quadruplex sequences in all primate lentiviruses corroborates the idea that these non-canonical nucleic acid structures are crucial elements in the lentiviral biology and thus have been selected for during evolution. In this work we aimed at investigating the presence and conservation of G-quadruplexes in the Retroviridae family. Genome-wide bioinformatics analysis showed that, despite their documented high genetic variability, most retroviruses contain highly conserved putative G-quadruplex forming sequences in their promoter region. Biophysical and biomolecular assays proved that these sequences actually fold into G-quadruplex in physiological concentration of relevant cations and that they are further stabilized by ligands. These results validate the relevance of G-quadruplexes in retroviruses and endorse the employment of G-quadruplex ligands as innovative antiretroviral drugs. This study indicates new possible pathways in the management of retroviral infections in humans and animal species. Moreover, it may shed light on the mechanism and functions of retrovirus genomes and derived transposable elements in the human genome.