Transduction 



335 



is, sensitive to lambda. This phage can be 

 collected from a culture of lysogenic bac- 

 teria in great quantities a few hours after a 

 brief exposure of these cells to ultraviolet 

 light. Such UV -induction causes the pro- 

 phage to replicate, and the progeny phage 

 lyse the cell. Using A phage collected from 

 lysogenic cultures, one finds that only a very 

 limited number of different bacterial mark- 

 ers can be transduced. They are restricted 

 to a region that controls galactose fermen- 

 tation, the Gal locus, whose markers are 

 known from conjugation studies to be very 

 closely linked. Lambda is therefore capable 

 only of restricted or specialized transduction. 



As mentioned, lysis of a lysogen and the 

 consequent liberation of infective phage can 

 be induced by ultraviolet light. When lyso- 

 genic Hfr conjugate with sensitive F~, a 

 number of zygotes are induced to lyse and 

 liberate infective phage. Initiated by con- 

 jugation this method of inducing prophage 

 to produce infective phage and lysis, is 

 called zygotic induction. Moreover, zygotic 

 induction by A occurs, with a given Hfr 

 strain, at a specific time after the start of 

 mating. This precise timing suggests that 

 the chromosome has a locus with which 

 lambda prophage is physically associated 

 during lysogeny. Jn a nonlysogenic cell, 

 no prophage is attached to or associated 

 with this site. When the site with the pro- 

 phage enters a sensitive F~ cell, zygotic in- 

 duction occurs. When crosses are made 

 between nonlysogenic Hfr (without pro- 

 phage) and lysogenic F^ (with prophage), 

 however, zygotic induction does not occur, 

 and the nonlysogenic locus is transferred and 

 segregates in the heterogenotes just as any 

 other genetic marker. From these results 

 and others, the locus for lambda prophage 

 maintenance is found to be closely linked to 

 the Gal locus which lambda can transduce 

 (see Figure 24-5, p. 324, in which the at- 

 tachment locus is given as A). 



The original lambda-containing lysogenic 



K12 bacterium is stable and haploid with 

 aspect to the Gal locus and produces only 

 about one <7<//-carrying lambda per 10 (! -10 7 

 phage. A lysate made by inducing such a 

 lysogens is capable of producing only a low 

 frequency of transduction ( LFT ) . Some of 

 the cells transduced by an LFT lysate form 

 clones unstable with respect to Gal, that is. 

 that segregate out cells with the Gal geno- 

 type of the recipient cell. In other words 

 a Gal' bacterium, transduced by an LFT 

 lysate of lambda carrying Gal+ , is usually 

 an unstable heterogenote being diploid and 

 heterozygous for Gal and occasionally seg- 

 regating Gal- progeny. The merozygote 

 produced by lambda transduction differs 

 from that produced by P22 in an abortive 

 transduction. In the latter case, the trans- 

 duced segment cannot replicate; in the for- 

 mer case, the transduced segment can rep- 

 licate, so that clones of merozygotes can 

 be produced. When infective lambda is in- 

 duced from a lysogenic host merozygotic for 

 Gal, the lysate contains one hundred times 

 as many phage which carry a Gal locus as 

 does the lysate of haploids. Such a Gal- 

 rich crop of phage is capable of a high fre- 

 quency of transduction (HFT). 



In the case of phages capable of gener- 

 alized transduction, transducing phage can 

 be obtained from the lysate of sensitive cells 

 infected with free phage; for phage capable 

 only of restricted transduction, transducing 

 phage cannot be obtained this way. There- 

 fore, in lambda, transducing phage are not 

 found in the lysate of infected nonlysogen- 

 ics and are released only from lysogenic 

 (haploid or merozygotic) bacteria." 



By employing different multiplicities and 

 combinations of transducing lambda (col- 

 lected from lysogenic cells) and nontrans- 

 ducing lambda (collected soon after nonly- 

 sogenic cells are infected), it is possible to 



K The preceding account is based largely upon the 

 work of J. Lederberg. E. M. Lederberg, and Nf. 1 . 

 Morse, and of E. L. Wollman and F. Jacob. 



