Crystal structure of a three-tetrad, parallel, K+-stabilized human telomeric G-quadruplex at 1.35†Šresolution.

The crystal structure of the G-rich human telomeric DNA Tel22 has been determined at 1.35 Šresolution in space group P6. Tel22 forms a non-canonical DNA structure called the G-quadruplex. The space group and unit-cell parameters are comparable to those in the crystal structures with PDB codes 6ip3 (1.40 Šresolution) and 1kf1 (2.15 Šresolution). The G-quadruplexes are highly similar in all of the structures. However, this structure of Tel22 displays clear density for polyethylene glycol and two potassium ions, which are located outside the ion channel in the G-quadruplex and play an important role in stabilizing the crystal contacts. In addition, 111 water molecules were identified (compared with 79 and 68 in PDB entries 6ip3 and 1kf1, respectively) that participate in intricate and extensive networks providing high stability to the G-quadruplex.

G-quadruplex dynamics contribute to regulation of mitochondrial gene expression.

Single-stranded DNA or RNA sequences rich in guanine (G) can adopt non-canonical structures known as G-quadruplexes (G4). Mitochondrial DNA (mtDNA) sequences that are predicted to form G4 are enriched on the heavy-strand and have been associated with formation of deletion breakpoints. Increasing evidence supports the ability of mtDNA to form G4 in cancer cells; however, the functional roles of G4 structures in regulating mitochondrial nucleic acid homeostasis in non-cancerous cells remain unclear. Here, we demonstrate by live cell imaging that the G4-ligand RHPS4 localizes primarily to mitochondria at low doses. We find that low doses of RHPS4 do not induce a nuclear DNA damage response but do cause an acute inhibition of mitochondrial transcript elongation, leading to respiratory complex depletion. We also observe that RHPS4 interferes with mtDNA levels or synthesis both in cells and isolated mitochondria. Importantly, a mtDNA variant that increases G4 stability and anti-parallel G4-forming character shows a stronger respiratory defect in response to RHPS4, supporting the conclusion that mitochondrial sensitivity to RHPS4 is G4-mediated. Taken together, our results indicate a direct role for G4 perturbation in mitochondrial genome replication, transcription processivity, and respiratory function in normal cells.