378 



CHAPTER 41 



contents of two or more phage particles, one 

 particle furnishing the exogenote and another 

 causing lysogeny. Using different concentra- 

 tions of phage, and low multiplicities es- 

 pecially, it was possible to prove that only one 

 phage particle is needed per transduction. It 

 was shown,^ moreover, that a single phage 

 which attacks a susceptible bacterium can 

 produce only one of three mutually exclusive 

 effects on its host, namely, death (by lysis 

 some 60 minutes later), lysogeny, or trans- 

 duction. A phage that produces generalized, 

 complete or abortive, transduction is, there- 

 fore, defective with regard to its phage 

 genotype and is subsequently unable to repli- 

 cate, at least when it is the only phage of that 

 type infecting the cell. What has happened 

 is that a small chromosome segment of the 

 last host bacterium must have replaced a 

 particular segment of the phage genome 

 whose presence is necessary for the subse- 

 quent lysis or lysogenization of a new host. 

 We had already anticipated this possibility 

 (p. 374). We cannot tell in the present case, 

 however, whether the DNA of the exogenote 

 is separate from, or attached to, the defective 

 phage genome. 



E. coli strain K-12 is normally lysogenic 

 for the temperate phage lambda (X), to which 

 it is relatively insensitive. A mutant strain 

 of E. coli was found which is sensitive to 

 lambda, that is, which is nonlysogenic. Since 

 the nonlysogenic strain exposed to lambda is 

 often lysed, it can be used to test for the 

 presence of lambda in a filtrate. Sometimes, 

 the sensitive strain becomes lysogenized. 

 This permits the study of the transducing 

 capacity of lambda by using a lysogenic 

 strain with different genetic markers from 

 those of the sensitive, nonlysogenic, strain. 

 The strains and procedure, then, are comp- 

 arable to those used to study transduction by 

 P22 in Salmonella. Lambda to be tested for 

 transducing ability can be collected from the 

 filtrate of the lysogenic strain. Lambda can 

 ^ By J. N. Adams and S. E. Luria. 



also be harvested, in greater quantity, a few 

 hours following a short treatment of lyso- 

 genics with uUraviolet light. Such UV- 

 induction causes prophage to replicate prog- 

 eny phage which lyse the cell. When the 

 transduceability of various markers is tested, 

 it is found that only a very limited range of 

 markers can be carried by lambda. These 

 are restricted to a cluster of loci. Gal, con- 

 troUing galactose fermentation, loci which, 

 from conjugation studies, are known to be 

 very closely linked to each other. Lambda is 

 therefore capable only of restricted transduc- 

 tion. 



Lysis, and the consequent liberation of 

 infective phage, can be induced by ultra- 

 violet light, as mentioned. It is also found 

 that when lysogenic Hfr conjugate with sensi- 

 tive F~, a number of zygotes are induced 

 to lyse and liberate infective phage. This 

 method of inducing prophage to replicate and 

 liberate infective phage by lysis, initiated by 

 conjugation, is called zygotic induction. It is 

 found, moreover, that zygotic induction oc- 

 curs, with a given Hfr strain, only if about 

 26 minutes of conjugation takes place before 

 it is interrupted. This suggests that the 

 chromosome has a locus, Lp (or /;-), with 

 which prophage is physically associated. In 

 a nonlysogenic cell there is no prophage 

 attached to or associated with the Lp site, 

 whereas in a lysogenic cell there is. More- 

 over, in crosses between nonlysogenic Hfr 

 {Lp without prophage) and lysogenic F~ cells 

 {Lp with prophage), there is no zygotic induc- 

 tion, and the nonlysogenic trait {Lp without 

 prophage) is transferred and segregates (cf. 

 p. 353) among recombinants just as any other 

 genetic marker. From these results and 

 others, it is found that Lp, the locus for 

 lambda prophage maintenance, is closely 

 linked to the Gal loci which lambda may 

 subsequently transduce (see Figure 39-4, page 

 361, where the locus under discussion is given 

 as X). 



The original lambda-containing lysogenic 



