DNA Base Sequence Changes Research Articles

Frameshift mutagenesis of lambda prophage by 9-aminoacridine, proflavin and ICR-191.

The changes in DNA base sequence induced in the lambda cI gene in an E. coli lysogen have been determined following mutagenesis by three acridine derivatives: 9-aminoacridine and proflavin, which bind reversibly to DNA; and ICR-191, which attaches covalently to DNA through a half-mustard group. For all three derivatives, most mutations are +1 and -1 frameshifts in runs of adjacent G:C pairs. The specificity of mutagenesis at various sites is similar for all three compounds. Prophage in mutL host cells, deficient in mismatch repair, are much more susceptible to mutagenesis by 9-aminoacridine. The induced mutations are also frameshifts, and the site specificity is the same as in lysogens of wild type cells. Thus, additions or deletions of single bases can be corrected by the mismatch repair system, but mismatch repair does not play an important role in determining the sequence specificity of the mutational events.

Changes in DNA base sequence induced by targeted mutagenesis of lambda phage by ultraviolet light

In targeted mutagenesis of lambda phage by ultraviolet light, the mutations are caused by radiation-induced lesions in the phage DNA. Of 62 mutations in the lambda cI gene that were sequenced, 41 (63%) of the targeted mutations were transitions, with similar numbers of C · G to T · A and T · A to C · G base changes. The remaining 21 mutations were about equally divided among eight transversions, seven frameshifts (5 additions and 2 deletions), and six double events with either two nearby base changes or a base change and a nearby frameshift. Of the 62 mutations, 60 could be associated with -Pyr-Pyr- sequences in the DNA, sites of likely photoproducts. For more information on this point, lambda phage were irradiated with 313 nm light in the presence of acetophenone, for which the major photoproduct is reported to be the thymine-thymine cyclobutyl dimer, with no measurable Pyr(6-4)Pyo photoproducts. Of 22 mutations sequenced, 19 were transversions and only one was a transition, permitting the conclusion that thymine-thymine cyclobutyl dimers are not the primary cause of ultraviolet light-induced transitions. A consideration of all the data strongly suggests that Pyr(6-4)Pyo photoproducts are mutagenic lesions.

DNA base sequence changes induced by bromouracil mutagenesis of lambda phage

The base sequence changes induced by bromouracil mutagenesis in the cI gene of phage lambda have been determined by direct sequence analysis. Phage DNA mutagenized during prophage replication or during phage lytic growth showed predominantly A · T → G · C transitions. The frequency of this mutation was strongly sequence-dependent: 5′ A-C-G-C 3′ > A-C(A.C or T) > A(A.G or T). The difference in mutability of bases in the gene is not the result of specificity in mutL-dependent mismatch repair, since phage grown in mutL host cells showed the same distribution of bromouracil mutations. The observations made in phage mutagenized with bromouracil in the prophage state should be representative of bromouracil mutagenesis in the Escherichia coli chromosome.

Mutagenesis in S49 mouse lymphoma cells: induction of resistance to ouabain, 6-thioguanine, and dibutyryl cyclic AMP.

The effects of mutagens on three genetic markers--resistance to ouabain, 6-thioguanine, and dibutyryl cyclic AMP (Bt2cAMP), were investigated in a mouse lymphoma cell line, S49. Nitrosoguanidine, ethyl methanesulfonate, ICR 191, and x-rays were used. Mutagen-specific responses were seen. Ouabain resistance was induced by nitrosoguanidine, but not by ICR 191. ICR 191 induced resistance to 6-thioguanine more efficiently than did nitrosoguanidine; the converse was true of resistance to Bt2cAMP. The relative frequency of biochemically distinguishable subtypes of mutants resistant to Bt2cAMP was characteristic of the mutagen used to generate them. The results can be interpreted as follows: nitrosoguanidine and ethyl methanesulfonate frequently, but ICR 191 and x-rays rarely, give rise to DNA base sequence changes that result in structurally altered but functional proteins. This type of change is required for induction of mutants resistant to ouabain and of certain classes of mutants resistant to Bt2cAMP. Resistance to 6-thioguanine and other classes of mutants resistant to Bt2cAMP can result from DNA base sequence changes that lead to extensive alteration of protein structure or expression; these changes are induced by ICR 191 or x-rays.