Introduction xxi 



(82) also discovered the existence of zygotic induction, described here in their 

 second paper. For if a chromosome fragment of a donor bacterium bearing a 

 prophage enters a non-lysogenic recipient cell, then the prophage becomes 

 induced, enters the vegetative state, leading to the production of infective 

 progeny and lysis and loss of the bacterial zygote. This phenomenon accounted 

 for the discrepancies that had been observed in earlier attempts to determine 

 the chromosomal location of the prophage by bacterial conjugation experi- 

 ments, from which diflFerent linkage relations could be inferred, depending 

 upon which of the two parents of the cross carried the prophage (160). 



A completely independent confirmation of the specific location of the pro- 

 phage on the bacterial chromosome was provided through transduction 

 experiments by Jacob (78), whose paper is presented here. Transducing 

 virus particles carry a small genetic segment of closely linked loci of a donor 

 bacterium, i.e. the last host cell, into a recipient bacterium, i.e. the next host 

 cell. In this way, recombinant bacteria can arise which have derived a very 

 limited part of their genome from the donor cell (164, 163, 147, 98). Jacob 

 thus showed that a transducing virus can carry the prophage of an entirely 

 unrelated virus strain from a lysogenic donor into a non-lysogenic recipient 

 bacterium, usually in association with the contiguous region of the donor 

 chromosome. The chromosomal region, moreover, turned out to be the same 

 in which the prophage had been already placed by bacterial conjugation 

 studies. 



The presence of the prophage not only endows the bacterium with the 

 capacity to produce phage but also confers upon the cell an immunity to 

 infection by a homologous phage (8, 18, 121, 157). Such virus particles are 

 usually adsorbed to immune lysogenic bacteria, but the particles neither 

 multiply to give rise to infective progeny nor afiFect growth and division of the 

 immune cells in any way ( 16, 15, 81 ) . The immune character is highly specific, 

 in that a bacterium lysogenic for, and hence immune to, infection by one 

 type of phage is not immune to infection by other viruses whose prophage 

 the cell does not happen to carry. It became possible to study the genetic 

 basis of immunity when Wollman and Jacob (84) discovered a number of 

 related phage strains that undergo genetic recombination with one another 

 but that differ in their immune specificity as well as in the locations of their 

 prophages on the host linkage map. The final paper of our collection presents 

 the work of Kaiser and Jacob (87) which established by means of crosses of 

 these related phage strains that there is a definite segment of the viral genome, 

 the C region, which determines the immune specificity of the phage. This 

 same segment also controls the ability of the virus to establish itself as pro- 

 phage and the locus at which the prophage is situated on the bacterial chromo- 

 some. Later experiments by Jacob and Campbell (80) have shown that im- 

 mimity derives from the presence of a repressor substance in the cytoplasm 

 of the lysogenic cell. The specificity of both formation and action of this 

 repressor appears to be determined by the C region of the phage genome. 



