252 ERNST FREESE 



\ . Ml TAGEMC Specificity 



The previous chapters have shown that I'ach mutagen has eertain 

 chemical properties and therefore is expected to iiuhK-e only certain base 

 pair changes to the exclusion of others. This chemical sjiecificity should 

 correspond to a mutagenic specificity such that each mutagen can 

 induce only certain of the possible mutations to the exclusion of others. 

 Some selection rules should be strict ami exceptions impossible, others 

 should merely indicate a more or less pronounced mutagenic preference 

 of a given mutagen. 



A mutagenic specificity has been actually observed for the induction 

 of reverse mutations by various mutagens. For example, Demerec 

 (1953) reported two mutants in E. coli of which one (leucine-dependent) 

 responds little to UV and much to MnClj while the other (phenylala- 

 nine-dependent) responds in the opposite way. In Xeurospora crassa 

 several adenine-dependent mutants, controlling the same functional 

 property, have been found to differ in their spontaneous and X-ray- 

 induced revertibility (Giles, 1955; De Serres. 1958) and can be dis- 

 tinguished by their pattern of response to different chemical mutagens 

 (Westergaard. 1957). A ver>' detailed analysis of mutagenic specificity 

 has been undertaken for the rll region of bacteriophage T4 which will 

 be reported in more detail below. Several results with bacterial systems 

 (Demerec. 1960; Rudner. 1960. 1961; Kirchner. 1960; Margolin and 

 Mukai, 1961; StrelzofT, 1962) and with yeast (Leuppold. 1962; Gutz. 

 1961) corroborate and extend these findings. 



A. THE SPECIFICITY OF FORW.\RD MUTATIONS 



Each functional property is determined by a genetic region, contain- 

 ing many sites whose change results in the loss of the function. It is not 

 surprising, therefore, that forward mutations for different functional 

 properties can be induced with similar frequency (same order of magni- 

 tude), because individual site specificities average out. But if one 

 examines the frequency of forward mutations at one particular site 

 quite a different picture emerges. For such an analysis a large number of 

 independently arising mutant* of the same functional property must be 

 isolated and mapped. The results for the ;•// mutants of phage T4 show 

 that mutations frequently recur at certain sites (hot spots). The loca- 

 tion of these sites is specific and different for different mutagens. The 

 highest specificity has been observed with spontaneous mutations (Ben- 

 zer, 1957, 19611, next with the base analogs 5-bromouracil (Benzer and 

 Freese, 1958) and 2-aminopurine (Freese, 1959a), and witli some in vitro 

 chemicals (Freese, 1959c; Benzer, 1961). Proflavin-induced mutational 



