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Cyclobutane-type pyrimidine dimers generated by ultraviolet irradiation of DNA can be cleaved by DNA photolyase. The enzyme-catalysed reaction is believed to be initiated by the light-induced transfer of an electron from the anionic FADH ) chromophore of the enzyme to the pyrimidine dimer. In this contribution, first infrared experiments using a novel E109A mutant ofEscherichia coliDNA photolyase, which is catalytically active but unable to bind the second cofactor methenyltetrahydrofolate, are described | ềFEBS Journal Light-induced reactions of Escherichia coli DNA photolyase monitored by Fourier transform infrared spectroscopy Erik Schleicher1 Benedikt HeBling2 Viktoria Illarionova1 Adelbert Bacher1 Stefan Weber3 Gerald Richter1 t and Klaus Gerwert2 1 Lehrstuhl fur Organische Chemie und Biochemie Technische Universitat Munchen Germany 2 Lehrstuhlfur Biophysik Ruhr-Universitat-Bochum Germany 3 Freie Universitat Berlin Fachbereich Physik Berlin Germany Keywords DNA photolyase DNA repair FT-IR pyrimidine dimer stable-isotope labelling Correspondence G. Richter Schoolof Biologicaland ChemicalSciences University of Exeter Stocker Rd Exeter EX4 4QD UK Fax 44 1392 26 3434 Tel 44 1392 26 3494 E-mail g.richter@exeter.ac.uk K. Gerwert Lehrstuhlfur Biophysik Ruhr-Universitat-Bochum Universitatsstr. 150 44780 Bochum Germany Fax 49 2343 21 4238 Tel 49 2343 22 4461 E-mail gerwert@bph.ruhr-uni-bochum Present address Freie Universitat Berlin Fachbereich Physik Arnimallee 14 14195 Berlin Germany tPresent address School of Biological and Chemical Sciences University of Exeter UK Received 9 December 2004 revised 10 February 2005 accepted 16 February 2005 doi 10.1111 j.1742-4658.2005.04617.x Cyclobutane-type pyrimidine dimers generated by ultraviolet irradiation of DNA can be cleaved by DNA photolyase. The enzyme-catalysed reaction is believed to be initiated by the light-induced transfer of an electron from the anionic FADH chromophore of the enzyme to the pyrimidine dimer. In this contribution first infrared experiments using a novel E109A mutant of Escherichia coli DNA photolyase which is catalytically active but unable to bind the second cofactor methenyltetrahydrofolate are described. A stable blue-coloured form of the enzyme carrying a neutral FADH radical cofactor can be interpreted as an intermediate analogue of the light-driven DNA repair reaction and can be reduced to the enzymatically active FADH form by red-light irradiation. Difference Fourier transform infrared FT-IR .