The Molecular Basis of Mutation 



397 



whereas intrachain dimerization interferes 

 with the proper base-pairing of T with A, 

 leading eventually to the formation of incor- 

 rect complements. 



Dimerization may explain how UV de- 

 stroys the primer-template activity of single- 

 stranded DNA, causes mutations in </>X174, 

 and destroys transforming DNA. UV radia- 

 tion has two opposite effects, however, de- 

 pending upon the wavelength employed: 

 At 2800A UV radiation tends to form di- 

 mers from monomers, whereas at 2400A 

 it tends to form monomers from dimers. In 

 fact, DNA inactivated as primer-template 

 by 2800A UV radiation is partially restored 

 to activity by subsequent exposure to 2400A 

 radiation. Similarly, the transforming ac- 

 tivity of Hemophilus DNA inactivated by 

 2800A can be partially reactivated by subse- 

 quent irradiation at 2390A. With large 

 doses of 2800A, about 50% of the biological 

 inactivation — as measured by transforming 

 ability — can be attributed to T dimer for- 

 mation, one inactivating "hit" being equiva- 

 lent to one dimer formed for each 160 nu- 

 cleotides. 10 



The replicational consequences of intra- 

 strand dimer formation can be studied in 

 vitro} 1 After various single-stranded DNA 

 primer-templates are exposed to 2800A ra- 

 diation, the products of synthesis are sub- 

 jected to nearest-neighbor analysis. As ex- 

 pected, the frequency of the AA sequence 

 decreases in proportion to the TT sequences 

 dimerized; the dinucleotide sequences con- 

 taining G, especially GG, increase in fre- 

 quency. These results strongly indicate that 

 T dimers in vivo decrease the chance that 

 the complementary AA sequence will incor- 

 porate opposite a TT sequence in the tem- 

 plate and suggest — but do not prove — that 

 these A's are often replaced by G's. 



'" As found by R. B. Setlow and J. K. Setlow 



(1962). 



11 As shown by R. B. Setlow. W. L. Carrier, and 



F. J. Bollum (1963). 



As mentioned, photorecovery from UV- 

 induced dimer formation occurs after expo- 

 sure to UV radiation of shorter wavelength. 

 In the presence of light of certain longer 

 wavelengths — blue light — a particular en- 

 zyme system has been found which can break 

 T dimers- — including those which are inter- 

 chain — into monoroers. Such a case illus- 

 trates chemoplwtorecovery (p. 191). Since 

 recovery from a mutagenic UV treatment is 

 only about 50%, UV radiation probably 

 produces mutations in other ways than dimer 

 formation. Since large doses of UV can 

 cause breaks in the DNA backbone in vitro, 

 this effect is probably another mutagenic 

 pathway in vivo. 



Although thymine dimers block DNA syn- 

 thesis in vitro and in vivo, certain strains of 

 E. coli are UV-radiation-resistant and even 

 in the dark can recover to resume DNA syn- 

 thesis. Such a recovery in these cells does 

 not involve the splitting of thymine dimers; 

 instead, the dimers are, by some mechanism, 

 removed from the DNA (the acid-insoluble 

 fraction) and appear in the acid-soluble 

 fraction. In an irradiated, sensitive strain, 

 which cannot synthesize DNA in the dark, 

 the dimers remain in the insoluble phase and 

 remain photorecoverable. 1 - Other work 13 

 suggests that intrastrand thymine dimers are 

 removed from the DNA of resistant cells 

 enzymatically, and that corrected DNA is 

 reconstructed from information on the com- 

 plementary strand. Such an error-correcting 

 mechanism would be biologically important 

 for the preservation of DNA. 



Finally, it should be noted that although 

 dimerization of 5-bromo uracil is difficult, 

 if not impossible, uracil dimers can be made 

 by UV irradiation. Consequently. UV is 

 expected to be mutagenic to RNA by the 

 same mechanisms as it is to DNA. 



Tautomeric shifts, physical and chemical 



12 See R. B. Setlow and W. L. Carrier (1964). 



13 See R. P. Boyce and P. Howard-Flanders 

 (1964). 



