Spore photoproduct

In direct repair the chemical modification that constitutes the lesions is reversed without removing and replacing nucleotides. There are four known direct repair enzymes photolyase, spore photoproduct lyase, methylguanine DNA methyltransferase, and AUcB family oxidative demethylases (1). 

Spore Photoproduct Lyase. The DNA in spores is in A-form because of dehydration. As a consequence, when spores are exposed to UV, the stereochemistry of the bases is not conducive to the formation of cyclobutane pyrimidine dimers or (6-4) photoproducts. Instead, UV induces the formation of 5-thyminyl-5,6-dihydrothymine or spore photoproduct (SP). This lesion is repaired by a 40-kDa enzyme called SP lyase. The enzyme is an iron-sulfur [4Fe-4S] protein that employs S-adenosyl-methionine (AdoMet) as a catalytic cofactor and carries out repair by a radical mechanism (6) (Fig. 2). In this mechanism, the reduced [Fe-S] center cleaves AdoMet to generate a 5 -deoxyadenosyl radical intermediate and methionine. The radical then abstracts an H-atom from C-6 of the SP. The resulting substrate radical undergoes bond cleavage to generate a product radical. The latter abstracts an H-atom from the 5 deoxyadenosine to form canonical thymines and a 5 deoxyadenosyl radical. Finally, the catalytic cycle is closed by electron transfer back to the [Fe-S] cluster concomitant with the formation of AdoMet (6). 

Figure 2 Reaction mechanism of spore photoproduct iyase. Spore photoproduct iyase removes the UV damage spore photoproduct. The enzyme is an iron suifur protein that uses AdoMet as a cofactor and repairs the damage through a radicai mechanism.

Buis JM, Cheek J, Kalliri E, Broderick JB. Characterization of an active spore photoproduct lyase, a DNA repair enzyme in the radical S-Adenosylmethionine Superfamily. J. Biol Chem. 2006 281 25994-26003. 

Exposure of DNA to UV radiation also generates oxidative base damage. In particular, thymine glycol (5,6-dihydroxydihydro-thymine) can be generated by saturation of the 5,6-double bond of some pyrimidines. Under anhydrous conditions, the so-called spore photoproduct or 5,6-dihydro-5-(a-thyminyl)-thymine, another type of dimeric pyrimidine photoproduct, can be generated by adduction of the methyl group of a thymine to the C5... 

UV radiation can result in the cross-finking of DNA to proteins (1). Crosslinks between different duplex DNA molecules (DNA-DNA crosslinks) have also been observed, mainly when DNA is irradiated in the dry state, in an extremely densely packed condition (such as in the heads of sahnon sperm), or in other special conformations (1). The spore photoproduct, formed in UV-irradiated dry DNA as described above, can arise not only between adjacent pyrimidines in the same chain but as a reaction between pyrimidines in different DNA chains (1). 

Knowledge about the enzyme structure is usually a prerequisite to set up a quantum chemical model and investigate the reaction mechanism. However, there are cases where the energetic feasibility of reaction mechanisms can be evaluated by studying individual steps without information about the structures. One example is the study of pyruvate-form ate lyase (PFL), where the calculations were able to support one of the suggested mechanisms before the X-ray crystal structure was solved." Another example is the study of the reaction mechanism of spore-photoproduct lyase (SPL), for which the crystal structure still remains to be solved. ... 

Figure 8 Reactions catalyzed by the radical-SAM superfamily. A. Lysine 2,3-aminomutase. B. Activating enzymes. C. Biotin synthase. D. Lipoic acid synthase. E. Spore photoproduct lyase.

Spore photoproduct lyase (SPL) catalyzes the repair of UV-induced DNA damage in Bacillus, and possibly other spore-forming microorganisms (Figure It has been shown to contain... 

In contrast to these examples is lysine aminomutase, in which the only significant cluster form observed is the [4Fe-4S] form, although a [3Fe-4S] cluster can be generated upon oxidation. However, no [2Fe-2S] cluster has been reported for this enzyme. LAM is also the only member of this family in which a [4Fe-4S] " cluster has been observed. The iron-sulfur cluster in spore photoproduct lyase has not been characterized, although some evidence for [4Fe-4S] and [3Fe S] forms has been obtained. ... 

The Chemistry of Lesion Formation Cyclobutane Pyrimidine Dimers (6- ) Photolesions The Dewar Valence Isomer The Spore Photoproduct Other DNA Photoproducts. 141 2... 

Photoinduced DNA Repair Cyclobutane Pyrimidine Dimer DNA Photolyases The Mechanism of Action of (6—4) Photolyase The Spore Photoproduct Lyase. 141 7... 

SCHEME 5 UV-light-induced formation of the spore photoproduct in spore DNA by an intrastrand or interstrand... 

SCHEME 6 Attempt to prepare the spore photoproduct by irradiation of chemically synthesized precursors, (a) Irradiation of thymine in a thin dry film, (b) Irradiation of a thiophenyl-substituted thymidine, which yields an intermediate 5-(2 -deoxyuridilyl)methyl radical, (c) Irradiation of a thymidylyl-thymidine dinucleotide. 

FIGURE 141.9 Proposed mechanism of action of the spore photoproduct lyase, which specifically repairs UV-induced spore DNA lesions. 

Pedraza-Reyes, M., Gutierrez-Corona, E, and Nicholson, W.L., Temporal regulation and forespore-specific expression of the spore photoproduct lyase gene by o-G RNA-polymerase during Bacillus subtilis sporulation, /. Bacterial., 176, 3983,1994. 

Fajardo-Cavazos, R, Salazar, C., and Nicholson, W.L., Molecular cloning and characterization of the Bacilllus subtilis spore photoproduct lyase (sp/) gene, which is involved in repair of UV radiation-induced DNA damage during spore germination, /. Bacteriol., 175, 1735, 1993. 

Rebeil, R. and Nicholson, W.L., The subunit structure and catalytic mechanism of the Bacillus subtilis DNA repair enzyme spore photoproduct lyase, Proc. Natl. Acad. Sci. USA, 98, 9038, 2001. 

Cheek, J. and Broderick, J.B., Direct H atom abstraction from spore photoproduct C-6 initiates DNA repair in the reaction catalyzed by spore photoproduct lyase evidence for a reversibly generated adenosyl radical intermediate,/. Am. Chem. Soc., 124, 2860, 2002. 

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