346 F. JACOB AND E. L. WOLLMAN 



temperate phage and of the host cell. In addition, temperate phages may 

 be involved in the mechanisms of genetic exchange in bacteria. 



In 1952, it was reported by Zinder and Lederberg that, in Salmonella 

 typhimurium, genetic characters could be transferred from bacteria of one 

 strain acting as a donor to bacteria of another strain acting as a recipient 

 by exposing the recipient to culture filtrates of the donor. The agent present 

 in the preparations and responsible for the transfer of genetic characters 

 was later identified by Zinder (1953) as a temperate phage released by the 

 donor and active on the recipient. 



Transduction, therefore, is the mechanism by which a temperate phage 

 transports a fragment of genetic material from a donor bacterium, on which 

 it has multipUed, to a recipient bacterium, which it infects. Among the 

 recipient cells which survive infection, rare individuals acquire new genetic 

 properties. In most cases, transduction is not specific in the sense that any 

 character of the donor can be transmitted by a small fraction of the phage 

 particles (10~^ or 10"^). It is independent of lysogenization by the trans- 

 ducing particles, since lysogenic recipients can be transduced by clear mutants 

 imable to lysogenize (Zinder, 1953). It seems likely that, in the course of 

 phage maturation, in addition to the phage genetic material, smaU segments 

 of the host chromosome can be incorporated at random into the protein coat 

 of maturing phage particles. 



Since its original discovery in Salmonella typhimurium, transduction has 

 now been described in several bacterial strains of Salmonella (Zinder, 1955), 

 E. coli, and Shigella (Lennox, 1955). Transducing activity, however, is re- 

 stricted to a limited number of temperate phages active on a given strain. 



When characters are closely linked, they may be transduced simultane- 

 ously (Stocker et ah, 1953; Lennox, 1955). This method allows an accurate 

 analysis of the fine structure of small segments of the bacterial chromosome 

 (Demerec et al, 1955). In E. coli K12(A), the lysogenic character itself may 

 also be transduced, together with the closely linked character Gal by means 

 of unrelated transducing phages (Jacob, 1955). This not only confirms the 

 chromosomal location of the prophage, but also shows that a phage particle 

 may contain the entire genome of another unrelated phage. 



A completely different type of phage-mediated transfer of genetic char- 

 acters has been recently described by Morse and associates (1956). In E. coli 

 K12(A), prophage A is located near characters controlling galactose fermen- 

 tation (Gal). After induction by UV light of Gal+ lysogenic K12(A), a small 

 proportion (10~* or 10~') of the phages released may transmit the Gal^ 

 character upon lysogenization of a Gal' nonlysogenic K12. Lysogenic Gal^ 

 clones thus obtained exhibit a remarkable property: they segregate 

 Gal^ as well as Gal~ progeny; they therefore possess both Gal alleles 

 and for this reason have been called heterogenotes. When heterogenofces 



