Pyrimidine dimers in ultraviolet-irradiated DNA's

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DNA's of various compositions, labeled with radioactive cytosine or thymine, were irradiated with monochromatic ultraviolet light and analyzed by photochemical and chromatographic techniques for photoproducts. The dimers TT,† CT and CC were identified as primary photochemical products, and UT and UU as secondary products that result from the deamination of cytosine-containing dimers. All the dimers have similar but not identical photochemical properties. The relative efficiency of forming dimers per dinucleotide is in the order TT>CT>CC and the relative cross-sections for splitting them by short wavelengths are CCCT>TT=UT=UU. All the pyrimidine dimers have similar biochemical properties. They are monomerized by photoreactivating enzyme plus visible light, and they interfere with nuclease action and inhibit DNA synthesis in vitro.

The number of cytosine-containing dimers is comparable to TT. For example, in Escherichia coli DNA irradiated with low doses, only 60% of the dimers are TT, and in Micrococcus lysodeikticus DNA only 20% are TT. At high doses the proportion of TT is greater than at low doses.

The amounts of uracil that are observed as a result of irradiation of [3H]cytosine-labeled DNA are much less than the amounts of cytosine-containing dimers.

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