Quantitation of Cyclobutane Pyrimidine Dirner Formation in Double- and Single-Stranded DNA Fragments of Defined Sequence

Abstract

The distribution of cyclobutane pyrimidine dimers in defined sequences of ultraviolet light-irradiated DNA was determined. The results demonstrate that the extent of dimer formation at a potential dimer site is a function of the dose and reaches a steady-state level for all dimers at doses above $2000\ {\rm J}/{\rm m}^{2}$. The steady-state level is primarily dependent upon the composition of the dimer, varying from a maximum of about 10% dimer formation at sites of adjacent thymines to less than 1% for sites of adjacent cytosines. The extent of dimer formation is also affected by the two bases that immediately flank the potential dimer site as well as by longer-range sequence effects. The rates of dimer formation and the steady-state levels at most dimers are similar in single- and double-stranded DNA. The dose rate of irradiation does not affect the distribution of pyrimidine dimers over the range of $1.8-7.5\ {\rm J}/{\rm m}^{2}/{\rm sec}$. The implications of these observations for understanding mutation rates at different sites within a gene are discussed.