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Lesion selectivity in blockage of lambda exonuclease by DNA damage
1CIBA-GEIGY, Environmental Health Center, Farmington, CT 06032.
Abstract:
Various kinds of DNA damage block the 3' to 5' exonuclease action of both E. coli exonuclease III and T4 DNA polymerase. This study shows that a variety of DNA damage likewise inhibits DNA digestion by lambda exonuclease, a 5' to 3' exonuclease. The processive degradation of DNA by the enzyme is blocked if the substrate DNA is treated with ultraviolet irradiation, anthramycin, distamycin, or benzo[a]-pyrene diol epoxide. Furthermore, as with the 3' to 5' exonucleases, the enzyme stops at discrete sites which are different for different DNA damaging agents. On the other hand, digestion of treated DNA by lambda exonuclease is only transiently inhibited at guanine residues alkylated with the acridine mustard ICR-170. The enzyme does not bypass benzo[a]-pyrene diol epoxide or anthramycin lesions even after extensive incubation. While both benzo[a]-pyrene diol epoxide and ICR-170 alkylate the guanine N-7 position, only benzo[a]-pyrene diol epoxide also reacts with the guanine N-2 position in the minor groove of DNA. Anthramycin and distamycin bind exclusively to sites in the minor groove of DNA. Thus lambda exonuclease may be particularly sensitive to obstructions in the minor groove of DNA; alternatively, the enzyme may be blocked by some local helix distortion caused by these adducts, but not by alkylation at guanine N-7 sites.
Insights
DNA damage inhibits lambda exonuclease activity, a 5' to 3' exonuclease. The enzyme
Area of Science:
- Molecular Biology
- Biochemistry
- Enzymology
Background:
- DNA damage poses a significant threat to genomic integrity.
- Exonucleases play crucial roles in DNA repair and metabolism.
- Understanding how DNA damage affects exonuclease activity is vital for comprehending DNA processing.
Purpose of the Study:
- To investigate the impact of various DNA damaging agents on the activity of lambda exonuclease, a 5' to 3' exonuclease.
- To compare the inhibitory effects of DNA damage on lambda exonuclease with those on 3' to 5' exonucleases.
- To elucidate the specific mechanisms by which DNA adducts impede lambda exonuclease function.
Main Methods:
- Enzymatic digestion assays using lambda exonuclease on DNA treated with various damaging agents (UV irradiation, chemical adducts).
- Analysis of enzyme processivity and identification of stalling sites.
- Characterization of DNA adducts and their locations within the DNA structure.
Main Results:
- Lambda exonuclease processive degradation is inhibited by UV irradiation, anthramycin, distamycin, and benzo[a]-pyrene diol epoxide.
- The enzyme stalls at agent-specific sites, similar to 3' to 5' exonucleases.
- Inhibition is transient for ICR-170 alkylation but persistent for benzo[a]-pyrene diol epoxide and anthramycin adducts.
- Lambda exonuclease appears sensitive to DNA minor groove modifications.
Conclusions:
- DNA damage significantly impairs lambda exonuclease activity.
- The enzyme's sensitivity to minor groove adducts suggests a specific recognition mechanism.
- These findings contribute to understanding DNA repair pathways and the impact of genotoxic agents.