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In vitro exposures of isolated DNA to one of the two carcinogenic and mutagenic chemicals, diethylsulfate or dimethylsulfate, induces several kinds of physicochemical and morphological alterations. These changes are detectable by a variety of independent techniques. A fraction of DNA treated briefly with either of these two chemicals moves during velocity centrifugation experiments less rapidly than the bulk of control DNA and more rapidly through gels during electrophoresis. This apparent decrease in size is paralleled by the formation of large DNA aggregates with mobilities indicating molecular weights several times that of the untreated, control DNA. The presence of a basic protein in the incubation mixture increases the rate of formation of such complexes. the tendency of the alkylated DNA to bind to both biological and non-biological materials is reflected in the increased attachment of DNA to columns built with methyl-esterified serum albumin and in its quantitative retention on nitrocellulose filters. DNA exposed to dimethylsulfate decreases its density in CsCl gradients. A mixture of two or more DNAs of different densities exposed to this chemical produces an u.v.-absorbing band which is found in such gradients at an intermediate density. If the alkylation reaction is carried out in the presence of a protein, a portion of DNA bands at a density intermediate between the density of DNA and that of the protein, even in the presence of an ionic detergent in the gradient. Under the electron microscope the alkylated DNA shows multiple single-strand breaks and peeling-off whiskers of denatured DNA. Aggregates of DNA molecules become visible upon further incubation of DNA with the alkylating agent. We suggest that the DNA-DNA and DNA- protein complexes play an important role in the process of carcinogenesis and mutagenesis.

Transforming DNA exposed to either diethyl sulfate (diES) or dimethyl sulfate (diMS) is inactivated. The rate of inactivation depends on the marker tested and on the chemical used: diMS is more active than diES. Cotransformation of linked markers is similarly depressed. In contrast, there is a transient increase in the cotransformation of distant, unlinked markers. These observations indicate that some of the intermolecular complexes of transforming DNA created in the test tube by the treatment with diES and diMS are biologically active. Radioactively labeled DNA treated with diES or diMS changes its patterns of interaction with cellular surfaces that are characteristic of untreated DNA. A possibility is considered that such alterations in DNA-protein interactions as well as the ability of these alkylating agents to transpose fragments of chromosomal material may play an important role in the processes of mutagenesis and, especially, carcinogenesis.

Transforming DNA was exposed to either beta-propiolactone or 1,3-propane sultone and then used for transformation of competent bacteria to nutritional independence from tyrosine and tryptophan (linked markers) and leucine (an unlinked marker). The ability to transform was progressively lost by the DNA during incubation with either of these two chemicals. For all three markers the inactivation curve was biphasic, with a short period of rapid inactivation followed by one characterized by a much slower rate. The overall rate of inactivation was different for all three markers and presumably was related to the size of the marker. The decrease in the transforming activity was in part due to the slower rate of penetration of alkylated DNA through the cellular membrane and its inability to enter the recipient bacteria. This decrease in the rate of cellular uptake, even for DNA eventually destined to enter the cell, began almost immediately after its exposure to the chemical and ended up with an almost complete lack of recognition of the heavily alkylated DNA by the specific surface receptors of competent cells. Such DNA attached to sites on the surface of competent bacteria which were different from receptors specific for the untreated nucleic acid. This attachment was not followed by uptake of the altered DNA. Presence of albumin during the incubation with a carcinogen further increased the degree of inactivation, indicating that the artificial nucleoproteins produced under such conditions were less efficient in the transformation assay than was the naked DNA. Cotransfomration of close markers progressively decreased, beginning immediately after the start of incubation of DNA with the chemicals. Extensively alkylated DNA fractionated by sedimentation through sucrose density gradients showed a peculiar distribution of cotransforming activity for such markers; namely, molecules larger than the bulk of DNA ("megamolecules") showed less ability to transform the second marker than did some of the apparently smaller molecules which sedimented more slowly through the gradient. An increase in cotransformation of distant markers was evident in DNA molecules after a short exposure to an alkylating agent, but cotransformation of such markers was absent in DNA treated for longer periods. The observed changes in the transforming and cotransforming activities of the alkylated DNA can be explained by what is known about the physicochemistry of such DNA and in particular about the propensity of the alkylated and broken molecules to form complexes with themselves and with other macromolecules.