G-quadruplexes in Haloferax volcanii

The archaeal domain is a taxonomically rich component of microbial communities that inhabit a wide range of habitats on Earth, including the human body. Phylogenomic analyses have indicated that archaea represent the closest known relatives of eukaryotes, thus suggesting that eukaryotes may have evolved from an archaeal ancestor. G-quadruplex structures (G4), formed by guanine rich sequences, are among the most intensively studied local DNA/RNA structures and regulate key biological processes such as replication and gene expression. A bioinformatics analysis of the genome of the salt-loving archaea H. volcanii revealed a large number of potential G4 sequences (PQS). Biophysical analyses showed that a representative panel of these sequences form stable G4 structures under physiological conditions in vitro. In addition, immunofluorescence experiments using the G4-specific antibody, BG4, detected G4s in vivo at the single-cell level with super-resolution microscopy. Moreover, we directly visualized G4 in exponentially growing or stationary cells both at the DNA and RNA levels. G4s were also observed in the RNA and DNA of the hyperthermophile archaeon T. barophilus. Finally, we identified helicases potentially involved in G4 unfolding. Together, with H. volcanii as a new model, our work helps to fill the gap between bacteria and eukaryotic organisms for G4 studies and will aid in uncovering the evolutionary history of G4 structures in the tree of life. ### Competing Interest Statement The authors have declared no competing interest.