Sequence-dependent modulation of frameshift mutagenesis at NarI-derived mutation hot spots

The NarI sequence is known to be the strongest mutation hot spot for induce d frameshift mutagenesis. indeed, a single N-2-acetylaminofluorene (AAF) ad duct induces -2 frameshift mutations (5/-GGCG(AAF)CC --> 5'-GGCC) more than 10(7)-fold over background mutagenesis in Escherichia coli. The mechanism of induction of the frameshift mutation involves a two nucleotide primer-te mplate misalignment event during replication of the adduct-containing seque nce. The slipped mutagenic intermediate (SMI) that is thus formed is strong ly stabilised by the AAF residue, in order to understand the origin of the extreme susceptibility of this sequence to frameshift mutagenesis, we analy sed AAF-induced mutagenesis at sequences 5'-N(a)GCG(AAF)CN(b)-3' containing the core dinucleotide GCGC repeat present in the NarI sequence flanked by variable nucleotides N-a and N-b. The nature of nucleotide N-b was found to strongly modulate the frequency of induced -2 frameshift mutagenesis (up t o 30 to 50-fold), while little if any effect could be attributed to nucleot ide N-a. The induction of -2 frameshifts, regardless of nucleotides N-a and N-b, was found to be SOS-inducible but umu/DC-independent as previously fo und for the authentic NarI sequence. The NarI sequence (GGCGCC) and sequenc e TGCGCA (N-a = T, N-b = A) were found to be equally "hot" for -2 frameshif t mutation induction compared to the sequence AGCGCT where induced mutagene sis was 30 to 50-fold lower. The analysis of replication events using constructions containing a strand marker across from the adduct site allowed us to demonstrate that the large difference in -2 frameshift mutagenesis is due to an intrinsic difference in the propensity of these sequences to slip during replication. How the na ture of the nucleotide flanking the adduct on its 3'-side (N-b) differentia lly stabilises the SMI will be discussed in the light of recent structural data and theoretical models. (C) 1999 Academic Press.