Structure of a triple helical DNA with a triplex-duplex junction

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Extended purine sequences can form triplex DNA structures.
• Triplex DNA is implicated in gene expression, recombination, and antisense inhibition.
• Previous studies were limited to solution-based NMR, lacking crystallographic data.

Purpose of the Study:

• To report the first crystallographic study of a DNA segment containing triplexes and its junction with a duplex.
• To elucidate the structural characteristics of DNA triplexes at high resolution.
• To investigate the conformational changes and interactions within DNA triplexes.

Main Methods:

• Novel design of DNA sequences for crystallization.
• X-ray crystallography at 1.8 Å resolution.
• Analysis of base-pairing (Hoogsteen-type interactions) and backbone conformation.

Main Results:

• The crystal structure reveals the third strand's backbone is parallel to the purine-rich strand.
• Hoogsteen-type interactions form consecutive C(+).GC, BU.ABU, and C(+).GC triplets.
• The DNA triplex adopts a unique conformation, distinct from A- and B-forms, with altered phosphate backbone torsion angles and a narrowed minor groove.

Conclusions:

• This study provides the first crystallographic insights into DNA triplex structure.
• The observed structural features, including backbone modifications and minor groove narrowing, are likely sequence-specific.
• Understanding these structures may facilitate the development of novel gene-targeting therapies.