338 F. JACOB AND E. L. WOLLMAN 



after infection of sensitive cells with temperate phages, or among the sur- 

 vivors of normal lysogenic bacteria which have been exposed to heavy doses 

 of UV light (Jacob, 1950; Lwoff and Simiaovitch, 1951; Appleyard, 1954b; 

 Jacob and Wollman, 1956). In the cultures of such defective lysogenic bac- 

 teria, no infectious particles (or very few) are found. After exposure to UV 

 light, lysis of the culture may or may not be observed. When lysis occurs, its 

 characteristics are the same as with normal lysogenic cells, but phage par- 

 ticles are released by a very small fraction of the population, as low as 10~^ 

 or less. The lysogenic character of the defective bacteria is nevertheless 

 clearly demonstrated by the fact that they exhibit the same immunity 

 pattern as the homologous normal lysogenic bacteria. In such a system, the 

 expression of lysogeny remains lethal for the host but is not accompanied by 

 the release of infectious phage particles. If a defective lysogenic strain 

 were isolated from nature, its lysogenic character could well remain 

 unnoticed. 



In a defective strain, some lesion of the lysogenic system prevents the 

 formation of mature phage particles. This lesion is generally located on the 

 prophage and not on the bacterial component of the system, since defective 

 bacteria induced and then infected with a homologous phage support the 

 growth of this phage and release mature particles (Appleyard, 1954b; Jacob 

 and Wollman, 1956). When the phage used for superinfection is marked by a 

 variety of genetic characters not only may it be recognized that the defective 

 character is a genetic property of the prophage but this particular mutation 

 may actually be mapped on the prophage linkage group. Genetic analysis of 

 various defective strains indicates that a variety of prophage mutations may 

 be responsible for the defective character. Mutations at different loci appear 

 to interfere with different steps of phage development. After UV induction, 

 prophage development may be initiated, but the lesion prevents one of the 

 steps involved in the completion of mature particles. The rare infectious 

 particles released result from mutations of the defective allele to a normal 

 one (Jacob and Wollman, 1956). Among the few cases which have been 

 analyzed in detail, one impHes a block in the morphogenesis of the phage 

 particles. Although all recognizable materials of phage A appear to be pro- 

 duced after UV induction, one of the reactions leading to their assembling 

 into mature particles is missing. In another case, it is the synthesis of a 

 pooled constituent which appears to be altered, since upon infection of UV- 

 induced defective bacteria with a homologous phage, both normal and 

 "defective" particles (that is, particles able to lysogenize but not to reproduce) 

 are released (Appleyard, 1956), Defective lysogenic bacteria therefore offer 

 the remarkable situation of a phage genetic material which can be indefinitely 

 perpetuated only in the prophage state, since lethal alleles prevent its de- 

 velopment into infectious particles. 



