Triple-stranded polynucleotide helix containing only purine bases

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Evolution

Background:

• The genetic code relies on specific base pairing (purine-pyrimidine).
• Understanding the origins of this complementarity is key to deciphering early molecular evolution.
• The role of non-standard base pairing in early genetic systems is largely unknown.

Purpose of the Study:

• To determine the structure of a complex formed by polyadenylic acid and polyinosinic acid.
• To investigate the conformational possibilities of non-purine-pyrimidine base pairing in helical nucleic acid structures.
• To assess the plausibility of early genetic codes based on adenine-hypoxanthine pairing.

Main Methods:

• X-ray diffraction analysis was used to determine the three-dimensional structure of the nucleic acid complex.
• Crystallographic data were analyzed to elucidate the helical conformations and base interactions.
• Comparative structural analysis was performed with known RNA double helix structures.

Main Results:

• A complex of one polyadenylic acid chain and two polyinosinic acid chains was structurally characterized.
• The three coaxial helical chains adopted conformations similar to conventional RNA double helices.
• Absence of standard purine-pyrimidine pairing was observed, yet helical structures were maintained.
• Formation of hypoxanthine pairs in codon-anticodon interactions requires only minor conformational adjustments.

Conclusions:

• Non-standard base pairing, such as adenine-hypoxanthine, can support stable helical nucleic acid structures.
• The evolution from a primeval genetic code based on adenine-hypoxanthine to the contemporary purine-pyrimidine code is molecularly feasible.
• This structural data supports models of early genetic code evolution without significant molecular discontinuities.