SEQUENCE-DEPENDENT MODULATION OF NUCLEOTIDE EXCISION-REPAIR - THE EFFICIENCY OF THE INCISION REACTION IS INVERSELY CORRELATED WITH THE STABILITY OF THE PRE-INCISION UVRB-DNA COMPLEX

The UvrABC excinuclease is involved in the nucleotide excision; repair (NER) pathway. Sequence-dependent differences in repair efficiency ha ve been reported for many different lesions and it is often suggested that sites with poor repair contribute to the occurrence of mutation h ot spots. However, guanine bases modified by N-2-acetylaminofluorence (AAF) within the NarI site-(5'-G(1)G(2)CG(3)CC-3') are incised by the UvrABC excinuclease with different efficiencies in a pattern not corre lated with the potency of mutation induction. To gain insight into the mechanism of sequence-dependent modulation of NER, we analyzed the fo rmation, the structure and the stability of UvrB-DNA pre-incision comp lexes formed at all three positions of the AAF-modified NarI site. We show that the efficiency of release of UvrA(2) from specific UvrA(2)B- DNA complexes is sequence-dependent and that the efficiency of incisio n is inversely related to the stability of the pre-incision complex. W e propose that the preincision complex, [UvrB-DNA], when formed upon d issociation of UvrA(2), undergoes a conformational change (isomerizati on step) giving rise to an unstable but incision-competent complex tha t we call [UvrB-DNA]'. The [UvrB-DNA] complex is stable and unable to form an incision-competent complex with UvrC. As the release of UvrA(2 ), this isomerization step is sequence-dependent. Both steps contribut e to modulate NER efficiency. (C) 1997 Academic Press Limited.