Photoactivated DNA Nicking, Enzyme Inactivation, and Bacterial Inhibition by Sesquiterpenoid Phytoalexins from Cotton. Tzeli Julia Sun. Department of Biochemistry, Oklahoma State University, Stillwater, OK 74078-0454 U.S.A. Margaret Essenberg, and Ulrich Melcher. Department of Biochemistry, Oklahoma State University, Stillwater, OK 74078-0454 U.S.A. MPMI 2:139-147. Accepted 21 February 1989. Copyright 1989 The American Phytopathological Society.

When irradiated by the sun or by cool-white fluorescent lamps (300-700 nm), the phytoalexins 2,7-dihydroxycadalene (DHC) and lacinilene C induced single-strand breaks in plasmid pBR322 DNA. UV radiation of wavelengths 239 nm and 300 nm (near the absorbance maxima of DHC) was more effective than radiation of wavelengths 400, 500, and 600 nm in activating DHC to nick DNA. Oxygen was required for full DNA-nicking. Scavengers of reactive oxygen species and of free radicals inhibited the nicking of plasmid by DHC plus radiation. Single-strand breaking of double-stranded DNA by DHC plus radiation was neither nucleotide sequence specific nor base specific as revealed in a DNA sequencing gel. Some preferential cleavage near guanine residues was observed when photoactivated DHC acted on single-stranded DNA. Catalytic activities of deoxyribonuclease I and malate dehydrogenase were greatly reduced after incubation with DHC plus radiation. In the dark, 0.1 mM DHC only partially inhibited multiplication of Xanthomonas campestris pv. malvacearum, but when irradiated, this concentration of DHC was bactericidal. Radiation alone was only bacteriostatic. It was concluded that DHC is a photosensitizer.