Fluorescently labeled model DNA sequences for exonucleolytic sequencing

Area of Science:

• Biochemistry
• Molecular Biology
• Biotechnology

Background:

• Enzyme-catalyzed labeling of DNA with fluorescent dyes is crucial for various molecular biology applications.
• Current methods face challenges in achieving precise and efficient incorporation of fluorophore-bearing analogs into DNA templates.

Purpose of the Study:

• To develop and optimize methods for low-density labeling of DNA with fluorescent dyes using enzymatic approaches.
• To evaluate the efficiency of different fluorophore-bearing deoxynucleotide triphosphates (dNTPs) incorporation.
• To demonstrate the utility of synthetic DNA constructs for precise fluorescent labeling and exonucleolytic sequencing.

Main Methods:

• Polymerase Chain Reaction (PCR) using Taq DNA polymerase for labeling natural DNA templates with dye-dNTPs.
• Partial substitution of natural dNTPs with fluorophore-bearing analogs (rhodamine-green-5-dUTP, tetramethylrhodamine-4-dUTP, Cy5-dCTP).
• Exonucleolytic degradation using T7 DNA polymerase to determine incorporation efficiency.
• PCR using a thermostable 3'-->5' exonuclease-deficient Tgo DNA polymerase mutant for labeling synthetic DNA constructs with dye-dNTPs (rhodamine-green-X-dUTP, tetramethylrhodamine-6-dUTP, Cy5-dCTP).
• Fluorescence correlation spectroscopy (FCS) to determine catalytic cleavage constants and quantify incorporated nucleotides.
• Re-sequencing methodologies (mobility-shift gel electrophoresis, reversion-PCR) for confirming covalent and base-specific incorporation.

Main Results:

• The covalent incorporation efficiency of dye-dNTPs into natural DNA decreased in the order: rhodamine-green-5-dUTP > tetramethylrhodamine-4-dUTP > Cy5-dCTP.
• Exonucleolytic degradation of labeled natural DNA by T7 DNA polymerase was characterized by catalytic cleavage constants between 0.5 and 1.5 s⁻¹.
• Synthetic DNA constructs allowed for complete substitution and precise positioning of dye-dNTPs, with six out of six possible incorporations achieved for rhodamine-green-X-dUTP and tetramethylrhodamine-6-dUTP.
• Novel re-sequencing analysis confirmed the covalent and base-specific incorporation of fluorescent nucleotides.

Conclusions:

• Enzyme-catalyzed PCR enables efficient low-density labeling of DNA with fluorescent dyes.
• Synthetic DNA constructs offer a powerful platform for creating tailor-made nucleic acids with precisely positioned fluorescent labels.
• The developed methodology and analysis techniques are valuable for advancing exonucleolytic sequencing and other DNA analysis applications.