Figure 4

Relative frequencies of tetramer sequences as a function of UV-irradiation dose. (a) Sequences from TTTT to GTGT show the large range of decay constants. When several TT-dimers in the sequence exist, the possibilities to form T=T lesions lead to rapid decay of the relative frequency. (b) For combinations of thymine and cytosine bases in the tetramers there is always the possibility to form several CPD-lesions. There are clear indications for a bi-exponential behavior. (c) Relative frequencies for a selection of tetramer sequences containing a central TT dimer plotted as a function of UV-irradiation dose. When the TT-dimer is flanked by purines A or G the decay is much slower than for flanking C, where CPD-lesions are possible between all bases of the tetramer. The decay of GTTG is surprisingly slow, presumably due to the possible charge-transfer reaction between G and T, acting as an additional decay channel reducing damage frequency. (d) 2D-plot of the relative frequencies for all 256 tetramers. (e) and (f) expanded views for tetramers with a large fraction of fast decaying sequences (e) containing TT dimers and pyrimidine trimers. When GT or GC dimers are present (f) the decay is very slow.