336 



CHAPTER 25 



prove ' that transducing lambda is defective 

 for a portion of the lambda genome. The 

 (idl locus being transduced replaces a \ari- 

 ably-sized segmenl o( the lambda genome. 

 The defective GaZ-transducing lambda par- 

 ticle. A(/,e. retains certain phage properties 

 and loses others. \dg has lost the abilit\ 

 to replicate and produce infective phage 

 progeny and the prophage it forms must be 

 defective; the host cell infected with a sin- 

 gle Adg particle is never lysogeni/ed. A 

 cell infected by Adg, therefore, can be su- 

 perinfected with a nontransducing phage 

 whose additional presence makes the host 

 lysogenic and contributes a function which 

 permits the defective prophage to multiply 

 after induction. At the time of lysis of such 

 a doubly-infected cell, infective phages of 

 both nontransducing and transducing types 

 are liberated. This situation parallels that 

 already described for Salmonella which can- 

 not be lysed or lysogenized if infected by a 

 single transducing P22 phage, but which 

 shows one of these characteristics if the host 

 is also infected with one or more normal, 

 nontransducing P22 phage particles. 



Transformation does not ordinarily occur 

 in E. coli, probably because of difficulty in 

 DNA penetration. If the DNA of Adg were 

 isolated and somehow introduced into the 

 bacterium, however, one would expect this 

 DNA to behave as a transforming principle 

 with respect to the Gal locus. Even if DNA 

 does not penetrate E. coli by itself, it might 

 be capable of entering with infecting whole 

 phage. Indeed, this does occur ,n ; that is. 

 using nontransducing lambda as a carrier 

 or "helper." DNA isolated from Adg is ca- 

 pable of Gal transformation. 



From the discussion of nontransducing 

 P22 and lambda, it should be clear that 

 such temperate phages have two alternative 



'■> See W. Arber. G. Kellenberger. and J. Weigle 



(1957). and A. Campbell (1964). 



10 See A. D. Kaiser and D. S. Hogness (1960). 



pathways of action open to them upon in- 

 fecting a sensitive bacterium: the phage 

 either lyses or lysogenizes its bacterial host. 

 As clearlj shown in the case of lambda, the 

 infecting phage either remains in the cyto- 

 plasm where it replicates taster than the 

 chromosome and eventually lyses and liber- 

 ates progeny phage, or it integrates in the 

 chromosome where it resides as prophage 

 and is replicated as a regular chromosomal 

 marker. Accordingly, lambda and most 

 other temperate phages are episomes. 



What is the basis for the difference be- 

 tween the temperate phages capable of gen- 

 eralized and those capable of restricted 

 transduction? A restrictive-transducing 

 phage usually has a specific chromosomal 

 locus for attachment to the host chromo- 

 some, a generalized-transducing phage has 

 not. Assuming — correctly — that the phage 

 genome is nucleic acid, it can be suggested 

 that the nucleotide sequence held in com- 

 mon between prophage and chromosome is 

 shorter for the generalized-transducing 

 phage than it is for the specialized trans- 

 ducer. In this connection it is noteworthy 

 that evidence has been obtained 1! that a 

 portion of the lambda genome is homolo- 

 gous to the E. coli chromosome, as revealed 

 by the ability of their denatured DNAs to 

 base pair with each other. Several experi- 

 ments suggest that a prophage makes the 

 host cell immune to further infection by 

 homologous phage, by preventing not the 

 penetration of the DNA but its replication. 

 This action parallels the suppression of free- 

 F replication by integrated F. 



Transduction by temperate phages has 

 been found to occur also in Pseudomonas. 

 Vibrio. Staphylococcus, and Proteus, and it 

 would not be surprising to find transduc- 

 tion occurring in a wide variety of other 

 types of cells, including human. 



11 By D. B. Cowie and B. J. McCarthy, and by 

 \1. H. Green. 



