CONFORMATION AND DYNAMICS OF THE MISPAIR O-6-ETHYLGUANINE-THYMINE

Alkylating agents such as N-ethyl-N-nitroso compounds produce G:C to A :T transition mutations in DNA owing to the ethylation of O-6-guanine. It is assumed that these mutations occur on replication via mispairin g of ethylguanine with thymine rather than the normal cytosine. Such a mispair could be either very stable compared with the normal G:C base pair or it could be less stable than a normal G:C base pair but cause little disruption to the DNA, so that it is not easily recognized by repair enzymes. In this study, the stability and conformation of a dod ecamer containing O-6-ethylguanine:thymine base pairs has been compare d with that of a similar duplex containing a normal sequence. rising m olecular dynamics to study a duplex DNA sequence containing O-6-ethylg uanine:thymine base pairs, we find a normal Watson-Crick alignment of the mispairs. However, this conformation is stabilised by only one or two hydrogen bonds depending on whether the conformation of the ethyl group is either the cis,trans or the trans,trans orientation with resp ect to the N(1) of guanine. Analysis of other conformational features and flexibility has shown that it is difficult to distinguish behaviou r of the modified sequence with that of the normal sequence. Like the sequences containing O-6-ethylguanine:cytosine, the modified O-6-ethyl guanine:thymine sequences are slightly less stable than those containi ng the normal Watson-Crick G:C sequence. However, both O-6-cis,trans-e thylguanine:thymine and O-6-trans,trans-ethylguanine:thymine cause ver y little disruption to the DNA helices in contrast to the behaviour of O-6-ethylguanine:cytosine and thus the former modifications will be d ifficult to distinguish from normal sequences by repair enzymes.