The mutational specificity of furazolidone in the lacI gene of Escherichia coli

The mutational specificity of the 5-nitrofuran derivative furazolidone was determined in the lad gene of Escherichia coli. E. coli strain TC3960 (Delta uvrB. pKM101) was treated with 10 mu M furazolidone, yielding an induced mutation frequency of 30 times over the spontaneous frequency. Mutations from 88 furazolidone-induced mutants were analyzed by DNA sequencing: 74 were base substitutions, 7 were frameshift mutations, 3 were tandem base substitutions, 3 were complex mutations and 1 deletion was detected. The specificity of mutation was compared to that of furylfuramide (AF2). Differences were observed in both the site specificity and the mutagenic specificity of the two 5-nitrofuran derivatives. (1) Furazolidone-induced point mutations were observed at both G:C and A:T base pairs; 93% of AF2-induced point mutations were targeted to G:C sites. (2) At G:C sites approximately equal numbers of G:C --> T:A transversions and G:C --> A:T transitions were induced by furazolidone; AF2-induced G:C --> T:A transversions outnumbered G:C --> A:T transitions 76:49. (3) There was no observable preference for particular sequences of furazolidone-induced mutations; the prominent hotspots for AF2-induced G:C --> T:A transversions, G:C --> A:T transitions and -(G:C) frameshifts were at 5'-TGC-3' sequences in the lacI gene. (4) Furazolidone-induced frameshifts occurred at homopolymeric sequences suggesting that the mutations arose through a strand slippage mechanism; UF2-induced frameshifts occurred at a nonreiterated G:C base pair and could be templated, through formation of a palindrome, by a sequence 110 base pairs upstream from the site of mutation. The significant differences that we observe between the two spectra do not support the notion that structurally different 5-nitrofuran derivatives might react in a similar manner with DNA to produce premutational lesions with similar characteristics.