Chemical Carcinogens as Frameshift Mutagens: Salmonella DNA Sequence Sensitive to Mutagenesis by Polycyclic Carcinogens

Other investigators have shown that several polycyclic carcinogens are frameshift mutagens in Salmonella. Mutagenic potency of these compounds is assessed by ability to induce reversion of histidine-requiring frameshift mutants to prototrophy. One frameshift mutation in the histidinol dehydrogenase gene, hisD3052, is unusually sensitive to mutagenesis by certain polycyclic carcinogens. We find that the 3052 mutation is a -1 deletion, probably loss of a G· C pair from a DNA repeat of -G-G-G- -$\overset \cdot \to{\text{C}}$-$\overset \cdot \to{\text{C}}$-$\overset \cdot \to{\text{C}}$-. The polycyclic carcinogens tested (e.g., 2-nitroso- -G-C- fluorene) revert 3052 by deleting a -$\overset \cdot \to{\text{C}}$-$\overset \cdot \to{\text{G}}$- doublet from the -C-G-C-G-C-G-C-G- DNA sequence -$\overset \cdot \to{\text{G}}$-$\overset \cdot \to{\text{C}}$-$\overset \cdot \to{\text{G}}$-$\overset \cdot \to{\text{C}}$-$\overset \cdot \to{\text{G}}$-$\overset \cdot \to{\text{C}}$-$\overset \cdot \to{\text{G}}$-$\overset \cdot \to{\text{C}}$- which is close to the 3052 site. This rare mispairing-prone sequence represents the carcinogen-sensitive ``hotspot'' in 3052. The ICR compounds, noncarcinogenic intercalating agents, show a broader specificity of mutagenesis. Reversion with these mutagens occurs predominantly by two mechanisms: one identical to that of the polycyclic carcinogens, and the other by +1 additions in a third DNA tract narrowly separated from the 3052 site. The alkylating carcinogen N-methyl-N′-nitro-N-nitrosoguanidine appears to have a similar dual specificity. DNA base changes in 3052 revertants have been correlated with properties of histidinol dehydrogenase in crude extracts. This correlation of DNA base sequence with electrophoretic and other properties of the mutant proteins allows one to analyze easily the specificity of new mutagens that mutate this strain.