Base-change analysis of revertants of the hisD3052 allele in Salmonella typhimurium.

This report is an investigation of the specific sequence changes in the DNA of Salmonella hisD3052 revertants induced by a set of specific frameshift mutagens found in our diet. They include B[a]P, aflatoxin B1, and the cooked-food mutagens, IQ, MeIQ, and PhIP. The Salmonella DNA was cleaved with restriction enzymes Sau3A, EcoR1, and Alu1 to give a 620-bp fragment containing the hisD3052 site. The size-fractionated fragments were ligated to the bacteriophage vector M13mp8. After transformation into E. coli, the recombinants were screened with a nick-translated hisD+ gene probe, and the isolated single-stranded DNA was sequenced. All IQ (13), MeIQ (3), PhIP (5), and aflatoxin B1 (3) induced revertants isolated had a 2-base (-CG- dinucleotide) deletion situated 10 bases upstream from the original hisD3052 -C- deletion. In contrast, 9 of 24 revertants induced by B[a]P had extensive deletions varying from 8 to 26 nucleotides in length and located at various sites along a 45-base-pair sequence beginning at nucleotide 2085 of the his operon. The other 15 B[a]P-induced revertants had a -CG- deletion at the same location as the revertants induced by the other food mutagens. 7 spontaneous revertants were also analyzed; they showed 3 -CG- deletions, 1 insertion and 3 distinct deletions (varying from 2 to 11 bases in size). In total, 13 distinct base changes are described which lead to reversion of the hisD3052 mutation.