photolyases

Photolyases are flavoprotein enzymes that repair UV-induced DNA damage by a direct, light-driven mechanism called photoreactivation. They restore pyrimidine-pairing by reversing cyclobutane pyrimidine dimers (CPDs) and, in many enzymes, 6-4 photoproducts formed between adjacent pyrimidines, without introducing strand breaks or removing nucleotides.

Repair requires light to energize the catalytic cofactor flavin adenine dinucleotide in the reduced form (FADH−). The enzyme may also use antenna chromophores such as pterins or folates to harvest additional light energy, enabling efficient repair under various light conditions. Blue to near-UV light is typically sufficient to drive the reaction.

Distribution and biology: Photolyases are widespread in bacteria, archaea, fungi, and plants. Most vertebrates lack functional photolyases and rely on nucleotide excision repair to fix UV lesions. In organisms that possess them, photolyases are usually located in the cytoplasm or nucleus, and in photosynthetic cells may be found in chloroplasts or other organelles.

Types and genetics: There are two main classes, CPD photolyases and 6-4 photolyases, each targeting specific lesions. In bacteria and some fungi, genes such as phrA and phrB encode these enzymes, though nomenclature varies among taxa. Some organisms possess enzymes with overlapping activities or additional regulatory features.

Applications: In molecular biology, photolyases are used to study DNA repair and to re-activate UV-damaged DNA in vitro. They have potential biotechnological and therapeutic relevance, but practical clinical use remains limited.