146 CELL HEREDITY 



treatment with acridiiic orange. Tlu' /•' cells so produced, when rein- 

 fected w ith /' particles, become, u ith a xcrv high frequency, Hfr instead 

 of F^. This seems to indicate that some episomal material remained in 

 the chromosome of the F cell to attract the invading episome to its site. 

 The concept of the episome and facts revealing its properties are new. 

 Recently evidence has been gathered indicating that a factor causing bac- 

 teria to produce a to.xic substance (bacteriocin) and one conferring multi- 

 ple drug resistance also behave as episomes. It may be that particles such 

 as these are of general occurrence in all cells and must be considered as 

 a possible general mechanism of cell heredity. 



The phenomenon of bacterial mating has been used to analyze many 

 situations in E. coli; among the most interesting is the location of 

 prophage. Crosses of lysogenic ( X*") by lysogenic (X^) bacteria give only 

 X"*" progeny, just as nonlysogenic ( X~) by nonlysogenic ( X~) crosses give 

 X bacteria. Crosses of ///rX~ X F~X"^ bacteria yield both types in 

 a proportion that depends on the selected markers used in the cross. 

 Thus the lysogenic and nonlysogenic factors behave as alleles at a single 

 locus. This X locus is closely linked to the gal locus. In crosses be- 

 tween strains carrying mutants of X, e.g., F"'"X^ x F~X' , no doubly 

 lysogenic strains or sensitives are found, but each mutant appears among 

 the recombinants linked with gal. The doubly lysogenic condition, 

 which can be achieved by superinfection, is also inherited as a single 

 locus. In contrast, a cross of a X~ a^ strain by a X^ a^ strain, where 

 alpha (a) is an unrelated prophage, does give some sensitives, as would 

 be expected if the loci were separate. The a phage is, as a matter of 

 fact, linked to xyl rather than to gal. 



If a mating is made between an Hfr X"^ and an F~ X^ strain, an unex- 

 pected result is obtained. Every time the X prophage enters the F~, it is 

 induced, the F~ cell is lysed, and X phages are released. This phenom- 

 enon is called zygotic induction. It is probably caused by the fact that 

 when the prophage moves into the nonlysogenic F" cell it escapes from 

 the repression that constitutes the immunity of lysogenic bacteria. Ad- 

 vantage can be taken of zygotic induction to locate X in the bacterial 

 linkage group. Table 5.9 shows how the location of a number of linked 

 prophages determined in an Hfr nonlysogenic x F~ lysogenic cross cor- 

 relates with the frequency of zygotic induction in an Hfr lysogenic x F~ 

 nonlysogenic cross. Furthermore, the time of entrance of the prophage 

 can be determined as that after which agitation in the blendor will not 

 rescue the F~ from zygotic induction. This time is an inverse function 

 of location along the chromosome. 



There is also a .series of noninducible prophages carried by these bac- 

 teria in the region of the chromosome where the mal, xyi and mtl genes 



