Cultures of bacteria are exposed to increasing doses of UV radiation, plotted along the horizontal axis. Survival curve of bacteria exposed to UV radiation. Four of the complementation groups, or genes, encode proteins that play major rules in NER they are uvrA, uvrB, uvrCand uvrD. By collecting large numbers of such mutants and testing them for their ability to restore resistance to UV radiation in combination, complementation groups were identified. Mutant strains can be identified that are substantially more sensitive to UV radiation these are defective in the functions needed for UV- resistance, abbreviated uvr. coli cells are killed only at higher doses of UV radiation. As illustrated in Figure 7.13, wild type E. The genes encoding this repair function were discovered as mutants that are highly sensitive to UV damage, indicating that the mutants are defective in UV repair. Some of the best-characterized enzymes catalyzing this process are the UvrABC excinuclease and the UvrD helicase in E. NER occurs in almost all organisms examined. The common feature of damage that is repaired by nucleotide excision is that the modified nucleotides cause a significant distortion in the DNA helix. This repair system is used to remove pyrimidine dimers formed by UV radiation as well as nucleotides modified by bulky chemical adducts. In nucleotide excision repair, damaged bases are cut out within a string of nucleotides, and replaced with DNA as directed by the undamaged template strand. The methylated enzyme is no longer active, hence this has been referred to as a suicide mechanism for the enzyme. It then removes the methyl group, transferring it to an amino acid of the enzyme. coli, recognizes O6‑methylguanine in duplex DNA. For instance, the enzyme O6‑methylguanine methyltransferase, encoded by the adagene in E. coli.Ī second example of the reversal of damage is the removal of methyl groups. The photolyase enzyme has two subunits, which are encoded by the phrA and phrBgenes in E. However, the result is that the DNA structure has been returned to its state prior to damage by UV. ![]() Note that this is not formally the reverse of the reaction that formed the pyrimidine dimers, since energy from visible light is used to break the bonds between the pyrimidines, and no UV radiation is released. The enzyme photolyase binds to a pyrimidine dimer and catalyzes a second photochemical reaction (this time using visible light) that breaks the cyclobutane ring and reforms the two adjacent thymidylates in DNA. Photoreactivation is a light-dependent process used by bacteria to reverse pyrimidine dimers formed by UV radiation. This occurs by specific enzyme systems recognizing the altered base and breaking bonds to remove the adduct or change the base back to its normal structure. Some kinds of covalent alteration to bases in DNA can be directly reversed.
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