Inhibition of spontaneous mutagenesis by vanillin and cinnamaldehyde in Escherichia coli: Dependence on recombinational repair

Abstract

Vanillin (VAN) and cinnamaldehyde (CIN) are dietary antimutagens that effectively inhibit both induced and spontaneous mutations. We have shown previously that VAN and CIN reduced the spontaneous mutant frequency in Salmonella TA104 (hisG428, rfa, DeltauvrB, pKM101) by approximately 50% and that both compounds significantly reduced mutations at GC sites but not at AT sites. Previous studies have suggested that VAN and CIN may reduce mutations in bacterial model systems by modulating DNA repair pathways, particularly by enhancing recombinational repair. To further explore the basis for inhibition of spontaneous mutation by VAN and CIN, we have determined the effects of these compounds on survival and mutant frequency in five Escherichia coli strains derived from the wild-type strain NR9102 with different DNA repair backgrounds. At nontoxic doses, both VAN and CIN significantly reduced mutant frequency in the wild-type strain NR9102, in the nucleotide excision repair-deficient strain NR11634 (uvrB), and in the recombination-proficient but SOS-deficient strain NR11475 (recA430). In contrast, in the recombination-deficient and SOS-deficient strain NR11317 (recA56), both VAN and CIN not only failed to inhibit the spontaneous mutant frequency but actually increased the mutant frequency. In the mismatch repair-defective strain NR9319 (mutL), only CIN was antimutagenic. Our results show that the antimutagenicity of VAN and CIN against spontaneous mutation required the RecA recombination function but was independent of the SOS and nucleotide excision repair pathways. Thus, we propose the counterintuitive notion that these antimutagens actually produce a type of DNA damage that elicits recombinational repair (but not mismatch, SOS, or nucleotide excision repair), which then repairs not only the damage induced by VAN and CIN but also other DNA damage-resulting in an antimutagenic effect on spontaneous mutation.

Fig. 1

Effects on survival and lacI mutant frequency of VAN (A) and CIN (B) in E. coli NR9102 (wild type). Single asterisks indicate VAN or CIN dose levels at which significant antimutagenic effects were observed (p < 0.001 in all indicated cases). Double asterisks indicate a toxic dose. Plotted are the natural log(ln) of the geometric means of surviving colonies per plate (n = 6) and the ln of the geometric means of the mutant frequency.

Fig. 2

Effects on survival and mutant frequency of VAN (A) and CIN (B) in NER-deficient E. coli NR11634 (uvrB); see legend to Fig. 1 for details.

Fig. 3

Effects on survival and mutant frequency of VAN (A) and CIN (B) in SOS-deficient E. coli NR11475 (recA430); see legend to Fig. 1 for details. Asterisk indicates a statistically significant antimutagenic effect (p < 0.001 for VAN at 40 μmol/plate; p < 0.05 for CIN at 10 μmol/plate).

Fig. 4

Effects on survival and mutant frequency of VAN (A) and CIN (B) in SOS-deficient and recombination-deficient E. coli NR11317 (recA56); see legend to Fig. 1 for details. Asterisk indicates a statistically significant mutagenic effect (p < 0.05 for CIN at 10 μmol/plate; p < 0.001 for CIN at 20 μmol/plate and VAN at 40 μmol/plate).

Fig. 5

Effects on survival and mutant frequency of VAN (A) and CIN (B) in MMR-deficient E. coli NR9319 (mutL); see legend to Fig. 1 for details. Asterisk indicates a significant mutagenic effect for VAN (p < 0.001 at 40 and 60 μmol/plate) and a significant antimutagenic effect for CIN (p < 0.05 for CIN at 10 μmol/plate).