V. MOLECULAR MECHANISM OF MUTATIONS 225 



the pi factor in E. coli (Hirota and Tijima, 1957; Hirota, 1960). Pro- 

 flavin prevents the assembly of phage T4 (De Mars et al., 1953) at 

 concentrations at which the uninfected bacterium can still duplicate and 

 even the phage DNA can still multiply. Certain dyes (methylene blue, 

 toluidene blue, and proflavin) when exposed to light have a strong lethal 

 effect on phages, provided they are inside the phage (Clifton, 1931; 

 Yamamoto, 1956, 1958; Kaufman and Hiatt, 1959; Hiatt, 1960). This 

 "photodynamic effect" presumably is due to the oxidation of some DNA 

 bases, mediated by the dye and stimulated by light; for the presence of 

 oxygen increases the effect and the presence of reducing agents (sulf- 

 hydryl groups) reduces it. 



A mutagenic effect so far has been reported only for acridine dyes. 

 Acrifiavin (a mixture of proflavin and 2,8-diamino-lO-methylacridinium 

 chloride) is mutagenic for bacteria (Witkin, 1947), and this as well as 

 other dyes are mutagenic for Allium (Bauch, 1948; D'Amato, 1950). 

 Proflavin is mutagenic for phages; when it is removed at some time after 

 phage infection normal phages are produced but an abnormally high 

 frequency of mutants is found (De Mars, 1953; Brenner et al., 1958). 

 Many other acridine dyes similarly show a mutagenic effect on phages 

 (Orgel and Brenner, 1961). 



The molecular mechanism by which acridine dyes induce mutations 

 is still unknown. Acridine orange and probably other acridines can stack 

 along nucleic acids. They apparently attach to the phosphate backbone 

 (Beers et al, 1958; Bradley and Wolf, 1959) except for polyadenylic 

 acid for which some attachment may involve the adenine bases (Steiner 

 and Beers, 1958). Lerman (1961) has recently found that proflavine 

 greatly increases the viscosity of DNA; from this and X-ray data he 

 concludes that acridines can be sandwiched between two purine bases 

 and thus cause some binding of the dye at concentrations at which the 

 stacking is still unimportant. With these data at hand and a particular 

 coding model in mind, Brenner et al. (1961) have proposed that acridines 

 induce only deletions or insertions of one base pair in DNA. 



D. CHEMICAL ALTERATION OF RESTING NUCLEIC ACID 



1. Nitrous Acid 



According to Schuster (1960) nitrous acid deaminates, with decreas- 

 ing frequency, the bases G, C, and A in DNA and according to Schuster 

 and Schramm (1958) with about equal frequencies in RNA. Its muta- 

 genic effect was first analyzed for tobacco mosaic virus and its RNA by 

 Mundry and Gierer (1958), for bacteria (Kaudewitz, 1959), phage T2 

 (Vielmetter and Wieder, 1959), and many other DNA- and RNA- 



