334 J. Phycol. 36, 334–341 (2000) EFFECTS OF TEMPERATURE ON THE PHOTOREACTIVATION OF ULTRAVIOLET-B–INDUCED DNA DAMAGE IN PALMARIA PALMATA (RHODOPHYTA) 1 Hans Pakker, Rute S. T. Martins, 2 Peter Boelen, Anita G. J. Buma Department of Marine Biology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands Osamu Nikaido Division of Radiation Biology, Faculty of Pharmaceutical Sciences, Kanazawa University, Kanazawa 920, Japan and Anneke M. Breeman 3 Department of Marine Biology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands The accumulation of DNA damage (thymine di- mers and 6-4 photoproducts) induced by ultraviolet-B radiation was studied in Palmaria palmata (L.) O. Kuntze under different light and temperature condi- tions, using specific monoclonal antibodies and sub- sequent chemiluminescent detection. Both types of damage were repaired much faster under ultraviolet-A radiation (UVAR) plus photosynthetically active radi- ation (PAR) than in darkness, which indicates photo- reactivating activity. At 12 ° C, all thymine dimers were repaired after 2 h irradiation with UVAR plus PAR, whereas 6-4 photoproducts were almost com- pletely repaired after 4 h. After 19 h of darkness, al- most complete repair of 6-4 photoproducts was found, and 67% of the thymine dimers were repaired. In a second set of experiments, repair of DNA dam- age under UVAR plus PAR was compared at three different temperatures (0, 12, and 25 ° C). Again, thymine dimers were repaired faster than 6-4 photo- products at all three temperatures. At 0 ° C, signifi- cant repair of thymine dimers was found but not of 6-4 photoproducts. Significant repair of both thym- ine dimers and 6-4 photoproducts occurred at 12 and 25 ° C. Optimal repair efficiency was found at 25 ° C for thymine dimers but at 12 ° C for 6-4 photo- products, which suggests that the two photorepair processes have different temperature characteris- tics. Key index words: dark repair; DNA damage; Palmaria palmata ; 6-4 photoproduct; photoreactivation; Rhodo- phyta; temperature; thymine dimer; UVAR; UVBR Abbreviations: UVAR, ultraviolet-A radiation; UVBR, ultraviolet-B radiation Stratospheric ozone depletion causes an increase of ultraviolet-B radiation (280–315 nm) to reach the earth’s surface. One of the main targets of UVBR in the cell is nuclear DNA. When UVBR is absorbed by DNA, many types of damage can occur, of which pyri- midine dimers are the predominant lesions (Strid et al. 1994). UVBR causes two adjacent pyrimidines in the same polynucleotide chain to bind covalently, which leads to a distortion of the DNA and conse- quently to an arrest of replication and transcription. Two types of pyrimidine dimers are recognized. The first are cyclobutyl pyrimidine dimers (CPDs), of which thymine dimers are the most abundant (ap- proximately 70% of all CPDs; Friedberg et al. 1995). Dimerization is achieved by a four-membered cyclo- butane ring (Friedberg et al. 1995). The second type of pyrimidine dimers are pyrimidine (6-4) pyrimidone photoproducts, or 6-4 photoproducts. Here, dimers are formed by a single bond between the C-6 and C-4 positions in adjacent pyrimidines (Friedberg et al. 1995). Both types of pyrimidine dimers can be de- tected using dimer-specific monoclonal antibodies and subsequent chemiluminescent detection (Vink et al. 1994). DNA damage can be repaired photoenzymatically in the presence of ultraviolet-A radiation (315–400 nm) and/or photosynthetically active radiation (PAR; 400–700 nm) (Strid et al. 1994, Britt 1995). This way of repair, known as photoreactivation or photoenzy- matic repair, reverses the dimer to its normal mono- meric form. The enzymes involved are photolyases, and specific photolyases have now been identified for thymine dimers (Sancar 1994) and 6-4 photoproducts (Todo et al. 1993, Chen et al. 1994). Although photo- reactivation appears to be a ubiquitous repair mecha- nism, there are several organisms, including diatoms and higher plants, in which it has not been found (Mitchell and Karentz 1993). A way of repair indepen- dent of light is nucleotide excision repair or “dark re- pair.” Several enzymes are involved in excising the damaged oligonucleotide and in replacing it by a newly synthesized fragment (Mitchell and Karentz 1993, Karentz 1994, Britt 1995). Organisms that dis- play efficient photoenzymatic repair appear to have a 1 Received 7 May 1999. Accepted 1 December 1999. 2 Present address: Center for Marine Sciences, University of Algarve, 8000 Faro, Portugal. 3 Author for reprint requests; e-mail breemana@biol.rug.nl.