Formation of dimers in ultraviolet-irradiated DNA.

Evidence has been obtained that in UV-irradiated native DNA, pyrimidine dimers are preferentially formed in long pyrimidine tracts. This effects is not the result of the existence of more dimerizable thymine in the long pyrimidine tracts. The preferential formation of dimers in long pyrimidine tracts is enhanced as the dose of irradiation is decreased. These results suggest that the formation of dimers in native DNA occurs by a cooperative mechanism. Quite likely, dimers are formed only in regions of native DNA that are locally denatured (breathing), in which the bases can be aligned. The formation of a dimer in such a breathing region would tend to lock it open and could lead to cooperative formation of dimers in such regions. If this mechanism for the cooperative formation of dimers is operative then it should be restricted to double-strand DNA. The pyrimidine tracts of irradiated single-stranded DNA all show virtually the same dimer content, irrespective of the length of the tract. These results are consistent with the hypothesis that dimers form cooperatively in breathing regions.