Jg. Levine et al., MUTATION SPECTRA OF GLU-P-1 IN SALMONELLA - INDUCTION OF HOTSPOT FRAMESHIFTS AND SITE-SPECIFIC BASE SUBSTITUTIONS, Environmental and molecular mutagenesis, 24(1), 1994, pp. 11-22
We used colony probe hybridization and PCR/ DNA sequence analysis to d
etermine the mutations in similar to 1,640 revertants of the -1 frames
hift allele hisD3052 and similar to 260 revertants of the base substit
ution allele hisG46 of Salmonella typhimurium induced by the heterocyc
lic amine cooked food mutagen 2-amino-6-methyldipyrido[1,2-a:3',2'-d]i
midazole (Glu-P-1). All of the mutations were at sites containing guan
ine, which is the base at which Glu-P-1 forms DNA adducts. A hotspot m
utation involving the deletion of a CG or GC within the sequence CGCGC
GCG accounted for 100% of the Glu-P-1-induced mutations at the framesh
ift allele in strains TA1978 (uvr(+)) and TA1538 (Delta uvrB) and 99%
in TA98 (Delta uvrB, pKM101). To explain the induction of these hotspo
t mutations by Glu-P-1, we describe here a more detailed version of ou
r recently proposed correct incorporation/slippage model [Genetics:136
:731, 1994]. We propose that after cytosine is incorporated correctly
opposite a Glu-P-1-adducted guanine, various slipped intermediates may
form (a total of 18), depending on which guanine is adducted and whet
her it remains within the helix or becomes extrahelical. This variety
of mutational pathways may account for the high mutability of the hots
pot sequence by Glu-P-1. Although the pKM101 plasmid does not influenc
e the mutagenic potency or mutational spectrum of Glu-P-1 at the frame
shift allele, it is required by Glu-P-1 to revert the base substitutio
n allele, where Glu-P-1 induces G.C --> T.A transversions (75%) and G.
C --> tA.T transitions (25%) exclusively at a single site (the second
position of the CCC codon of the hisG46 allele). The limited (20-30 ti
mes less) base substitution mutagenic potency of Glu-P-1 relative to i
ts frameshift mutagenic potency as well as the extreme site specificit
y exhibited by Glu-P-1 for base substitutions may have bearing on the
lack of base substitutions identified in ras genes in Glu-P-1-induced
rat colon tumors. (C) 1994 Wiley-Liss, Inc.