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Modern Genetic Analysis Chapter 7. Gene Mutations The Molecular Basis of Mutation
Table 7-3. Repair Systems in E. coli

General mode of operation | Example | Type of lesion repaired | Mechanism

Detoxification | Superoxide dismutase | Prevents formation of oxidative lesion | Converts peroxides into hydrogen peroxide, which is neutralized by catalase
Direct removal of lesions | Alkyltransferases | O-6-Alkylguanine | Transfers alkyl group from O-6-alkylguanine to cysteine residue on transferase
| Photolyase | 6-4 photoproduct | Breaks 64 bond and restores bases to normal
| Photolyase | UV photodimers | Splits dimers in the presence of white light
General excision | uvrABC-encoded exonuclease system | Lesions causing distortions in double helix, such as UV photoproducts and bulky chemical additions | Makes endonucleolytic cut on either side of lesion; resulting gap is repaired by DNA polymerase I and DNA ligase
Specific excision | AP endonucleases | AP sites | Makes endonucleolytic cut; exonuclease creates gap, which is repaired by DNA polymerase I and DNA ligase
| DNA glycosylases | Deaminated bases (uracil, hypoxanthine), certain methylated bases, ring-opened purines, oxydatively damaged bases, and certain other modified bases | Removes base, creating AP site, which is repaired by AP endonucleases
| GO system | 8-oxodG | A glycosylase removes 8-oxodG from DNA; another glycosylase converts any remaining 8-oxodG·A mispairs into 8-oxodG·C pairs, and the first glycoslyase then removes the 8-oxodG
Postreplication | Mismatch-repair system | Replication errors resulting in base-pair mismatches | Recognizes newly synthesized strand by detecting nonmethylated adenine residues in 5-GATC-3 sequences; then excises bases from the new strand when a mismatch is detected
| Recombinational repair | Lesions that block replication and result in single-stranded gaps | Recombinational exchange
| SOS system | Lesions that block replication | Allows replication bypass of blocking lesion, resulting in frequent mutations across from lesion

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