XX Introduction 



genie strain harbors and maintains a noninfeetive strueture, the probacterio- 

 phage or prophage, which endows the cell with the ability to give rise to infec- 

 tive phage without further intervention of exogenous virus particles. The 

 actual synthesis of infective phage, however, proceeds in only a small fraction 

 of the cells of a growing culture of lysogenic bacteria, whose intracellular 

 content of virus particles is liberated by lysis of the phage-producing indi- 

 vidual. LwofiF also inferred from these experiments that the induction of 

 phage development in a lysogenic cell is under control of external factors, 

 and his subsequent investigations in collaboration with Siminovitch and 

 Kjelgaard (118) showed that treatment with various agents, in particular 

 irradiation with UV light, will indeed induce phage production and ultimately 

 lysis of almost every cell of a culture of lysogenic bacteria. The first pre- 

 liminary report of this finding (117) is included in this collection. After the 

 publication of Lwoff's papers and reviews (114), the study of lysogeny not 

 only flowered into a distinct branch of bacterial virus research but also became 

 the bridge leading from the genetics of virus to that of host cell. In fact, the 

 recognition of the existence and nature of the proviriis state engendered 

 entirely new ideas concerning the origin, evolution, and biological function of 

 viruses (79). (Reviews: 114,17,85.) 



What is the relationship of the prophage to the remainder of the lysogenic 

 cell? The great stability of the lysogenic character implies that the prophage 

 is transmitted to daughter cells at each bacterial division. This could happen 

 in one of two ways: either the prophage represent? numerous autonomous 

 structures, replicating in the bacterial cytoplasm in synchrony with the rest of 

 the bacterium and being partitioned at random at each division over the 

 daughter cytoplasms, or the prophage is integrated into the nuclear apparatus 

 of the host cell and participates in the specific replication and segregation 

 process which assures, nolens volens, that each daughter cell obtains one 

 complete set of parental hereditary factors. The first of these alternatives soon 

 appeared unlikely, when indirect estimates of the number of prophages 

 revealed that each cell seems to carry only one or two prophages per bacterial 

 nucleus (16, 81), and that a given type of prophage appears to saturate a 

 limited number of sites on some bacterial structure (15). Positive indications 

 that the prophage is integrated into the bacterial nucleus became available 

 from experiments in bacterial conjugation, in which non-lysogenic bacteria 

 were crossed with lysogenic bacteria, and a linkage of the lysogeny character 

 with other known genetic factors of the cell inferred from the segregation 

 pattern of the recombinants (96, 158, 7, 55). After the discovery of high- 

 frequency-recombination (Hfr) bacterial strains and of the oriented transfer 

 of the bacterial chromosome from donor to recipient cell, Wollman and Jacob 

 ( 159 ) could show very clearly in their first paper of this collection that at least 

 one particular prophage has its specific location on the bacterial chromosome; 

 their later work, furthermore, revealed that different prophages have different 

 specific chromosomal sites (83, 84). At the same time, Jacob and Wollman 



