Norin 1, a progenitor of many economically important Japanese rice strains, is highly sensitive to the damaging effects of UVB radiation (wavelengths 290 to 320 nm). Norin 1 seedlings are deficient in photorepair of cyclobutane pyrimidine dimers. However, the molecular origin of this deficiency was not known and, because rice photolyase genes have not been cloned and sequenced, could not be determined by examining photolyase structural genes or upstream regulatory elements for mutations. We therefore used a photoflash approach, which showed that the deficiency in photorepair in vivo resulted from a functionally altered photolyase. These results were confirmed by studies with extracts, which showed that the Norin 1 photolyase-dimer complex was highly thermolabile relative to the wild-type Sasanishiki photolyase. This deficiency results from a structure/function alteration of photolyase rather than of nonspecific repair, photolytic, or regulatory elements. Thus, the molecular origin of this plant DNA repair deficiency, resulting from a spontaneously occurring mutation to UV radiation sensitivity, is defective photolyase.
INTRODUCTIONUV radiation can damage plants, decreasing growth and productivity (Teramura, 1983). UV radiation-augmentation studies have identified many UV radiation-sensitive cultivars of higher plants that are of economic importance, including rice (Kumagai and Sato, 1992;Bornman and Teramura, 1993;Hidema et al., 1996;Correia et al., 1998), and UV radiationexclusion studies indicate that prevention of such damage can increase plant growth (Krizek et al., 1997(Krizek et al., , 1998. UV radiation also induces photodamage in DNA, including damage to the cyclobutane pyrimidine dimer (CPD) and the (6-4)-pyrimidine-pyrimidone photodimer. Such damage can be lethal or mutagenic to simple and complex organisms; it can also impede replication and transcription, a possible mechanism for the adverse effects observed in higher plants.Photoreactivation is the major pathway in plants for repairing UV radiation-induced DNA damage (reviewed in Britt, 1996Britt, , 1999. This one-enzyme repair path is mediated by an enzyme, photolyase, which binds to a dimer to form a complex that is stable in the absence of light. When a photon in the wavelength range of 300 to 600 nm is absorbed (Saito and Werbin, 1969;Pang and Hays, 1991;Takeuchi et al., 1998), the dimer is reversed to two monomer pyrimidines and the enzyme is released. Photorepair of CPDs has been reported in several plant species, including gingko (Trosko and Mansour, 1969), Arabidopsis (Pang and Hays, 1991;Britt et al., 1993), alfalfa (Quaite et al., 1994b), soybean (Sutherland et al., 1996), cucumber (Takeuchi et al., 1996), rice (Hidema et al., 1997), maize (Stapleton and Walbot, 1994;, and wheat (Taylor, 1996).Increased sensitivity to UV radiation may result from failure to repair photodamage in DNA. The UV radiation-sensitive uvr1 mutant of Arabidopsis cannot photorepair (6-4)-pyrimidine-pyrimidone photodimers (Britt et al., 1993). Landry et al. (1997) s...