H-DNA

H-DNA. H-DNA is a specific, intramolecular triple helix structure that can form in DNA sequences containing mirror repeat symmetry, particularly those rich in purine and pyrimidine bases. This non-canonical conformation, a subset of triplex DNA, arises under conditions of negative supercoiling and low pH, where one strand of the duplex folds back to engage the major groove via Hoogsteen base pairing. Its formation is highly sequence-dependent and has been implicated in various genomic processes, including the regulation of gene expression and the potential to induce genetic instability.

Structure and formation

The fundamental architecture of H-DNA relies on sequences with mirror repeat symmetry, often long stretches of polypurine-polypyrimidine tracts. Under physiological stress, such as the torsional strain introduced by negative supercoiling, one half of the mirror repeat can unwind. A pyrimidine-rich strand from this region then dissociates and re-invades the major groove of the remaining duplex, forming a triple helix through Hoogsteen base pairing or reverse Hoogsteen base pairing with the Watson-Crick base pairs. This process creates a triplex region and leaves the complementary purine-rich strand displaced as a single-stranded loop, a structure sometimes referred to as a D-loop. The stability of this conformation is influenced by factors like divalent cation concentration and the presence of specific triplex-binding ligands. Key structural studies, including work by Alexander Rich and Maxim D. Frank-Kamenetskii, have elucidated its formation using techniques like X-ray crystallography and nuclear magnetic resonance.

Biological significance

H-DNA formation is not merely a structural curiosity but has significant functional consequences within the genome. Its presence in regulatory regions, such as promoter sequences and enhancer elements, can directly influence transcription by blocking the access of proteins like RNA polymerase or transcription factors, including members of the SP1 family. This structural switching provides a potential mechanism for the dynamic, topology-dependent control of gene expression. Furthermore, the single-stranded regions generated during H-DNA formation are susceptible to chemical modification and can form secondary structures like cruciform DNA, making these sites hotspots for mutagenesis and chromosomal translocations. Such instability is studied in contexts like Friedreich's ataxia and certain oncogenes. The structure may also play a role in chromatin organization and is investigated in relation to DNA repair pathways and recombination events.

Discovery and history

The concept of triple-stranded nucleic acids was first proposed in the 1950s by scientists like Gary Felsenfeld, but H-DNA was specifically identified and characterized in 1987. The pivotal discovery is credited to the laboratory of Alexander Rich at the Massachusetts Institute of Technology, with key contributions from researchers such as Vladimir Lyamichev and Maxim D. Frank-Kamenetskii. Their work, published in journals like *Nature* and the *PNAS*, demonstrated the formation of an intramolecular triplex in supercoiled plasmids containing mirror-repeat sequences. This built upon earlier observations of intermolecular triplexes by David A. Davies and others. The "H" in its name is derived from "hinge" DNA, reflecting its formation at a hinge point within the sequence, though it is also associated with the H-DNA of H-form DNA.

Related structures and variants

H-DNA is one member of a broader family of non-B DNA structures that deviate from the standard B-DNA double helix. It is closely related to other triple-helical forms, such as intermolecular triplex DNA and the G-quadruplex structures formed by guanine-rich sequences. Another variant includes parallel triplex DNA, where the third strand orientation differs. Other structurally distinct non-canonical forms that can coexist or compete with H-DNA in genomic regions include Z-DNA, a left-handed helical conformation stabilized by alternating purine-pyrimidine sequences, and the i-motif, a four-stranded structure formed by cytosine-rich strands under acidic conditions. The study of these structures often involves similar investigative teams, such as those led by Stephen Neidle and Thomas Cech.

Experimental detection and analysis

Detecting and characterizing H-DNA in vitro and in vivo requires specialized biophysical and biochemical techniques. Its formation in purified plasmid DNA is often probed using two-dimensional gel electrophoresis, which can resolve topological isomers, and enzymatic assays with structure-specific nucleases like SI nuclease or T7 endonuclease I. Spectroscopic methods, including circular dichroism and nuclear magnetic resonance, provide detailed information on base pairing and helical geometry. In cellular contexts, techniques such as in vivo chemical probing and the use of engineered triplex-binding ligands help map its formation. Advanced imaging approaches, including atomic force microscopy and studies utilizing the European Synchrotron Radiation Facility, have visualized these structures. Research from institutions like the National Institutes of Health and the Weizmann Institute of Science continues to refine these detection methodologies.

Category:DNA Category:Molecular biology Category:Nucleic acid structure