Lederberg: Genetics 



tribute samples into separate test tubes. Rep- 

 lica plating, transposing a pattern of surface 

 growth from plate to plate with a sheet of 

 velvet, takes the place of assaying inocula dis- 

 tributed in tubes (53). Dilution sampling and 

 replica plating are, then, alternative methods 

 of indirect selection whereby the test line is 

 spared direct contact with the drug. Selection 

 is accomplished by saving sublines whose sib- 

 ling clones show the resistant reaction. This 

 proof merely reinforces the incisive arguments 

 that had already been forwarded by many 

 other authors. 



If mutations are not specific responses to the 

 cellular environment, how do they arise? We 

 still have very little information on the proxi- 

 mate causes of spontaneous, even of radiation 

 and chemically induced, mutation. Most mu- 

 tagenic chemicals are potent alkylating agents, 

 e.g., formaldehyde or nitrogen mustard, which 

 attack a variety of reactive groups in the cell. 

 Similar compounds may occur in normal me- 

 tabolism and account for part of the spontane- 

 ous mutation rate; they may also play a role as 

 chemical intermediates in radiation effects. 

 For the most part, then, studies on mutagene- 

 sis, especially by the more vigorous reagents, 

 have told us little about the chemistry of the 

 gene. Probably any agent that can penetrate 

 to the chromosomes and have a localized 

 chemical effect is capable of introducing ran- 

 dom errors into the genetic information. If the 

 cell were not first killed by other mechanisms 

 most toxic agents would then probably be mu- 

 tagenic. 



Another class of mutagenic chemicals prom- 

 ises more information: analogues of the 

 natural nucleins which are incorporated into 

 DNA, For example, bromouracil specifically 

 replaces thymine in phage DNA when fur- 

 nished as bromodeoxyuridine to infected bac- 

 teria. Freese has shown, by genetic analyses of 

 the utmost refinement, that the loci of result- 

 ing mutations in T4 phage are distributed dif- 

 ferently from the mutants of spontaneous ori- 

 gin or those induced by other chemicals (18). 

 This method presumably maps the locations of 

 thymine in the original DNA. In order to ac- 

 count for wide variations in mutation rate for 

 different loci, further interactions among the 

 nucleotides must be supposed. So far, these 



studies represent the closest approach to a ra- 

 tional basis for chemical mutagenesis. How- 

 ever, every gene must present many targets to 

 any nuclein analogue and the specificity of 

 their mutagenesis can be detected only in sys- 

 tems where the resolution of genetic loci ap- 

 proximates the spacing of single nucleotides 

 (4) . At present this is feasible only in micro- 

 organisms; similar studies with bacteria and 

 fungi would be of the greatest interest. 



More specific effects might result from the 

 insertion of oligo- and polynucleotides, a pro- 

 gram which, however, faces a number of tech- 

 nical difficulties: even if the requisite polymers 

 were to be synthesized, there are obstacles to 

 their penetration into cells. The use of DNA 

 extracted from mutant bacteria to transfer the 

 corresponding genetic qualities is discussed as 

 "genetic transduction." 



RNA is the one other reagent that may be 

 expected to recognize particular genes. As yet 

 we have no direct evidence that the transfer of 

 information from DNA to RNA is reversible. 

 However, the anti-mutagenic effect of nuclein 

 ribosides (21, 71) may implicate RNA in mu- 

 tation. The reversibility of DNA «=^ RNA in- 

 formation is also implicit in Stent's closely 

 reasoned scheme for DNA replication (82). 

 The needed experiment is the transfer of 

 DNA information by some isolated RNA. Al- 

 though not reported, this has probably not 

 been fairly tried. 



One motivation for this approach is the diffi- 

 cult problem of finding sources of homogene- 

 ous nucleic acids. DNA occurs biologically as 

 sets of different molecules presumably in equi- 

 molar proportions. (A useful exception may 

 be a remarkably small phage which seems to 

 be unimolecular [85]). The species of RNA, 

 however, may vary with the predominant met- 

 abolic activity of the cells. If so, some molecu- 

 lar species may be sufficiently exaggerated in 

 specialized cells to facilitate their isolation. A 

 purified RNA would have many potential ap- 

 plications, among others as a vehicle for the 

 recognition of the corresponding DNA im- 

 plied by our theory of information transfer. 

 Pending such advances, specific mutagenesis 

 is an implausible expectation. 



Adaptive mutations, of which drug resist- 

 ance is a familiar example, are crucial to the 



S-70 



