The photoinduced cleavage of plasmid DNA by UV -A light in the presence of pterin was investigated. Electrophoretic analysis of the irradiated plasmid pUCI8 in the presence of pterin showed that UV light of 350 run induced the transformation of a significant proportion of the supercoiled plasmid to its relaxed form. A minor proportion of plasmid forms are also converted to the linear plasmid isomer at the longer irradiation times. All these transformations during irradiation can be observed in the absorption spectrum of DNA as function of time. Such spectral modifications correlated with the extent and the kinetics of plasmid relaxation, but not with the appearance of the linear plasmid. None of the!;e changes were operative without the irradiation with UV-A light. Control experiments with pterin or plasmid DNA irradiated separately, showed no photochemical changes. Results taken together suggest that the observed c'langes in the supercoi\ed plasmid as well as the spectral modifications both derive from the generation of single-strand break in the DNA . J ntroductionAs reported in the literature, pterins have been suspected as photoinducer of genetic alterations. The impOltance of this effect is mainly associated with the broad band absorption at wavelength larger than 320 nm . This fact makes relevant the effect of solar irradiation in the UV -A region on those biological systems where pterins could be accumulated. This suggestion was advanced in the last eighties (1).Several studies are reported in the literature describing the interaction of DNA with UV radiation. Dimers of pyrimidine are nonnal photoproducts resulting from this interaction (2). Such changes were observed in vitro and also in living organisms when DNA is exposed to UV-B (290-320 nm) radiation. Chemical changes in DNA induced by UV light result in mutagenic and carcinogenic alterations. These changes induce molecular processes competing with those involved in enzymatic reaction for repairing the DNA damage. If the later process is low enough, a genetic failure will be transmitted to the new cell (3) . It has been shown that UV-B radiation is able to induce several types of skin cancer (4).