Crystal structure of Z-DNA in complex with the polyamine putrescine and potassium cations at ultra-high resolution

Right-handed Z-DNA at ultrahigh resolution: a tale of two hands and the power of the crystallographic method

The self-complementary L-d(CGCGCG)₂ purine/pyrimidine hexanucleotide was crystallized in complex with the polyamine cadaverine and potassium cations. Since the oligonucleotide contained the enantiomeric 2'-deoxy-L-ribose, the Z-DNA duplex is right-handed, as confirmed by the ultrahigh-resolution crystal structure determined at 0.69 Å resolution. Although the X-ray diffraction data were collected at a very short wavelength (0.7085 Å), where the anomalous signal of the P and K atoms is very weak, the signal was sufficiently outstanding to clearly indicate the wrong hand when the structure was mistakenly solved assuming the presence of 2'-deoxy-D-ribose. The electron density clearly shows the entire cadaverinium dication, which has an occupancy of 0.53 and interacts with one Z-DNA duplex. The K⁺ cation, with an occupancy of 0.32, has an irregular coordination sphere that is formed by three OP atoms of two symmetry-related Z-DNA duplexes and one O5' hydroxyl O atom, and is completed by three water sites, one of which is twofold disordered. The K⁺ site is complemented by a partial water molecule, the hydrogen bonds of which have the same lengths as the K-O bonds. The sugar-phosphate backbone assumes two conformations, but the base pairs do not show any sign of disorder.

Machine learning shows torsion angle preferences in left-handed and right-handed quadruplex DNAs

Left-handed G quadruplexes (LHG4) have been recently discovered as a new class of G quadruplexes. The biological functions of LHG4s are still unknown, but they share a striking resemblance to Z-DNA in their helicity and jagged phosphate backbone. To further understand structural features of the LHG4s that define their left handedness, we have employed human-interpretable machine-learning methods to classify right- and left-handed G4s purely based on torsional angle analysis. Our results reveal the importance of the α, β, δ, and χ angles in left-handed structuring across both Z-DNAs and LHG4s. Our analysis may serve as the first step to understanding the conditions of formation for LHG4s and their potential biological relevance.

Non-canonical DNA structures: Diversity and disease association

A complete understanding of DNA double-helical structure discovered by James Watson and Francis Crick in 1953, unveil the importance and significance of DNA. For the last seven decades, this has been a leading light in the course of the development of modern biology and biomedical science. Apart from the predominant B-form, experimental shreds of evidence have revealed the existence of a sequence-dependent structural diversity, unusual non-canonical structures like hairpin, cruciform, Z-DNA, multistranded structures such as DNA triplex, G-quadruplex, i-motif forms, etc. The diversity in the DNA structure depends on various factors such as base sequence, ions, superhelical stress, and ligands. In response to these various factors, the polymorphism of DNA regulates various genes via different processes like replication, transcription, translation, and recombination. However, altered levels of gene expression are associated with many human genetic diseases including neurological disorders and cancer. These non-B-DNA structures are expected to play a key role in determining genetic stability, DNA damage and repair etc. The present review is a modest attempt to summarize the available literature, illustrating the occurrence of non-canonical structures at the molecular level in response to the environment and interaction with ligands and proteins. This would provide an insight to understand the biological functions of these unusual DNA structures and their recognition as potential therapeutic targets for diverse genetic diseases.