Conformational changes induced in DNA by the in vitro reaction with the mutagenic amine: 3-N,N-acetoxyacetylamino-4,6-dimethyldipyrido (1,2-a: 3', 2'-d) imidazole

Spectrophotometrical and immunochemical studies on the conformational changes in poly(dG-dC).poly(dG-dC) after modification by 4-hydroxyaminoquinoline 1-oxide

Poly(dG-dC).poly(dG-dC) was modified by the reaction with 4-hydroxyaminoquinoline 1-oxide (4HAQO) in the presence of seryl-AMP. The conformations of 4HAQO-modified poly(dG-dC).poly(dG-dC) and of poly(dG-dC).poly(dG-dC) were studied by circular dichroism spectra under various salt concentration conditions. 4HAQO residues to guanine bases are inefficient in inducing the transition of poly(dG-dC).poly(dG-dC) from B-form to Z-form conformation. We have elicited monoclonal antibodies against 4HAQO-poly(dG-dC).poly(dG-dC). They were characterized using enzyme-linked immunosorbent assay (ELISA), radioimmunoassay (RIA) and binding to supercoiled DNA. These antibodies reacted with 4HAQO-poly(dG-dC).poly(dG-dC) specifically but not with 4HAQO-modified DNA or poly(dG).poly(dC). However, they cross-reacted with N-acetoxy-2-acetylaminofluorene-modified poly(dG-dC).poly(dG-dC) in Z-form conformation. These monoclonal antibodies may recognize a unique conformation in poly(dG-dC).poly(dG-dC) after 4HAQO modification.

Activity of carcinogens that bind to the C8 position of guanine residues in an assay specific for the detection of -2 frameshift mutations in a defined hot spot

In this paper we describe a reversion assay specific for the detection of -2 frameshift mutations occurring within short stretches of alternating GC sequences. We have compared a series of chemical carcinogens that all bind covalently to the C8 position of guanine residues for their potency in inducing revertants in this assay. Large variations in potency are found within the list of compounds that were tested. The most potent chemicals tested induce the reversion frequency by a factor of 10(5) over background, whereas others only increase it by two orders of magnitude. These differences are discussed in terms of the conformational changes that the different C8-guanine adducts induce in DNA.

Induction of -2 frameshift mutations within alternating GC sequences by carcinogens that bind to the C8 position of guanine residues: development of a specific mutation assay

Using a forward mutation assay we have previously found that N-2-acetylaminofluorene (AAF), a strong chemical carcinogen, induces a majority of frameshift mutations located at specific sequences called mutation hot spots. Among these hot spot sequences, the NarI sequence (GGCGCC), is specific for -2 frameshifts (GGCGCC)----GGCC). Interestingly, these frameshift mutations occur independently of a functional umuDC locus. Being interested in elucidating this mutation pathway we have developed a reversion assay that is specific for this class of mutations. The assay is based on the reversion of a +2 frameshift mutant of plasmid pBR322 from tetracycline sensitivity to tetracycline resistance. It is shown that only "true" reversion events lead to tetracycline resistance. The carcinogen AAF induces this reversion event at a frequency that is increased four- to fivefold over the background frequency. A series of chemical carcinogens which, like AAF, bind covalently to the C8 position of guanine, are compared for their efficiency to induce this specific mutation event. Large variations in the mutagenic efficiency of these chemicals are observed and discussed in terms of the anti/syn conformation of the carcinogen-modified guanine residue. Based on this test, we describe a convenient spot assay that this presently used in our laboratory to isolate Escherichia coli mutants affected in this mutation pathway.

Comparison of the reactivity of B-DNA and Z-DNA with two isosteric chemical carcinogens: 2-N,N-acetoxyacetylaminofluorene and 3-N,N-acetoxyacetylamino-4,6-dimethyldipyrido-[1,2-a:3',2' -d] imidazole

The reactivity of nucleic acids in various conformations and two isosteric chemical carcinogens 2-N,N-acetoxyacetylaminofluorene (N-AcO-AAF) and 3-N,N-acetoxyacetylamino-4,6-dimethyldipyrido [1,2-a:3',2'-d] imidazole (N-AcO-AGlu-P-3) have been studied. Both carcinogens bind covalently to poly(dG-dC).poly(dG-dC) (B form) and to poly(dG-br5C).poly(dG-br5dC) (Z form). They also bind covalently to (dC-dG)16 and to (dG-dT)15 sequences inserted in plasmids when the inserts are in the B form but they do not bind to the inserts in the Z form. The reactivity of guanine residues at the B-Z junctions depends upon the superhelical density of the plasmids and upon the base sequences at the junction. The distribution of AGlu-P-3 modified guanines in a restriction fragment of pBR322 is not uniform and is different from that of AAF-modified guanines. The conclusion is that N-AcO-Glu-P-3 as N-AcO-AAF can probe at the nucleotide level the polymorphism of DNA. On the other hand, the non-reactivity of both chemical carcinogens and Z-DNA and the hyperreactivity of some junctions might have some importance in the understanding of chemical carcinogenesis.

In vitro enzymatic methylation of DNA modified with the mutagenic amine: 3-N,N-acetoxyacetylamino-4,6-dimethyldipyrido(1,2-a:3'2'-d )imidazole

Both the initial velocity and the overall methylation of DNA modified by acetylamino-4,6-dimethyldipyrido(1,2-a:3',2'-d)imidazole (A-Glu-P-3) by rat liver DNA-(cytosine-5-)-methyltransferase are decreased as compared to native DNA. A-Glu-P-3 bound to guanine residues may block the movement of the enzyme along the helix. The modified DNA does not inhibit the enzymatic methylation of native DNA. The enzyme has a lower affinity for the modified DNA than for native DNA. The hypomethylation caused by this carcinogen could have a significance in gene activity, cellular differentiation and cancer induction.

Left-handed Z-DNA

Since the Watson-Crick proposal of right-handed B-DNA, numerous studies have been devoted to the conformation of DNA. Both natural DNAs of heterogeneous sequences and synthetic DNAs are capable of adopting more than one conformation. The specific conformation a DNA adopts appears to depend mainly on its base sequence and its environmental conditions. For a given DNA, changes in environmental conditions can induce conformational transitions which occur according to cooperative or non-cooperative processes (for general reviews see Ref. 1a, b). Despite many results, molecular biologists did not put much emphasis on the polymorphism of DNA. The discovery of the intraconversion in helical sense between the right-handed B and left-handed Z conformers of DNA has brought a new interest in the polymorphism of DNA. It is now proposed that this polymorphism has important functions in biological reactions. A recent review, 'The Chemistry and Biology of Left-handed Z-DNA', by Rich et al. has just been published. We here report some of the results published in 1984 on Z-DNA.