Cytosine deamination in mismatched base pairs

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Cytosine deamination is a spontaneous DNA damage event.
• Mismatched base pairs can arise during DNA replication or repair.
• The rate of cytosine deamination in mismatched DNA is not well characterized.

Purpose of the Study:

• To quantify the rate of cytosine deamination in specific DNA mismatched base pairs (T.C and C.C).
• To compare deamination rates in mismatched versus matched DNA base pairs.
• To infer the structural state of mismatched DNA based on deamination kinetics.

Main Methods:

• Utilized M13mp2 heteroduplex DNA molecules with defined T.C or C.C mispairs.
• Incubated DNA molecules at 60°C and 37°C to measure deamination rates.
• Assessed deamination by observing plaque phenotypes in an ung- Escherichia coli host strain after transfection, correlating uracil formation with reversion frequencies.

Main Results:

• Hydrolytic deamination rate constants for T.C and C.C mispairs at 60°C ranged from 8 x 10⁻¹⁰ to 40 x 10⁻¹⁰ s⁻¹.
• At 37°C, rate constants for these mispairs were between 0.4 x 10⁻¹⁰ and 1.3 x 10⁻¹⁰ s⁻¹.
• Deamination rates in mismatched pairs were significantly higher (1-2 orders of magnitude) than in matched Watson-Crick G.C pairs, and comparable to single-stranded DNA.

Conclusions:

• Cytosine residues in T.C and C.C mispairs are highly susceptible to deamination.
• The elevated deamination rates suggest significant single-stranded character in these mismatched structures at physiological temperatures.
• Mismatched base pairs represent a vulnerable state for DNA, prone to mutations via cytosine deamination.