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The degree of ultraviolet light damage to DNA containing iododeoxyuridine or bromodeoxyuridine is dependent on the
1Molecular Science Group, Peter MacCallum Cancer Institute, Melbourne, Victoria, Australia.
Abstract:
The sequence selectivity of 300 nm ultraviolet light damage to DNA containing bromodeoxyuridine or iododeoxyuridine was examined on DNA sequencing gels. This was accomplished using a system where an M13 template was employed to direct synthesis of DNA in which thymidine was fully substituted with bromodeoxyuridine or iododeoxyuridine. The sites of damage corresponded to the positions of analogue incorporation. The extent of damage varied considerably at different sites of cleavage and ranged from the undetectable to over fifteen times the limit of detection (as assessed by laser densitometer scans). Strong damage sites had the "consensus" sequence CTT while sites of no detectable damage had the "consensus" sequence GTR. Bromodeoxyuridine and iododeoxyuridine had the same sites of damage although the extent of damage varied at different sites and bromodeoxyuridine damage was slightly greater than iododeoxyuridine. DNA containing thymidine was not damaged to any detectable level in this system with 300 nm ultraviolet light. The use of three closely related DNA sequences as targets for damage confirmed that (1) the sites of analogue incorporation are the cause of ultraviolet damage; and (2) that the neighbouring DNA sequence is an important parameter in determining the extent of damage. It is proposed that the microstructure of DNA--in particular the distance between the 5-carbon of the pyrimidine base (which is attached to the halogen) and hydrogen on the 2' carbon of the 5'-deoxyribose--ultimately determines the degree of cleavage with large distances giving a small degree of damage and smaller distances a large degree of damage.
Insights
Ultraviolet light damages DNA at sites where bromodeoxyuridine or iododeoxyuridine replace thymidine. DNA sequence and base structure influence the extent of this UV damage.
Area of Science:
- Molecular Biology
- Photochemistry
- Genetics
Background:
- Ultraviolet (UV) light can induce DNA damage.
- Incorporation of halogenated nucleoside analogs into DNA can alter its susceptibility to damage.
- Understanding sequence-specific UV damage is crucial for fields like photochemotherapy and DNA repair.
Purpose of the Study:
- To investigate the sequence selectivity of 300 nm UV light-induced DNA damage.
- To determine if bromodeoxyuridine (BrdU) or iododeoxyuridine (IdU) incorporation influences UV damage sites and extent.
- To explore the role of DNA sequence context and microstructure in UV-induced DNA cleavage.
Main Methods:
- Synthesis of M13 DNA templates with full thymidine substitution by BrdU or IdU.
- Exposure to 300 nm UV light.
- Analysis of DNA damage sites and extent using DNA sequencing gels and laser densitometry.
Main Results:
- UV damage occurred specifically at positions of BrdU or IdU incorporation.
- Damage extent varied significantly, with 'CTT' sequences showing strong damage and 'GTR' sequences showing none.
- BrdU and IdU exhibited similar damage sites, but BrdU damage was slightly greater.
- DNA containing only thymidine showed no detectable UV damage.
- Neighboring DNA sequences significantly influenced the extent of UV damage.
Conclusions:
- Halogenated nucleoside incorporation dictates UV damage sites.
- DNA sequence context and local DNA microstructure (e.g., base-to-deoxyribose distance) determine the degree of UV-induced DNA cleavage.
- 300 nm UV light selectively damages DNA containing BrdU or IdU, with sequence-dependent variations.