Bacteria: Recombination {III) 



359 



since E. coli is normally multinucleate. In 

 fact, such breakages are found to occur spon- 

 taneously also. Because of spontaneous 

 rupture, the spontaneous transfer of the Hfr 

 chromosome is usually partial, so that a piece 

 of variable size is injected into the recipient 

 cell. The resultant zygote of an Hfr cross is, 

 then, a partial diploid (as is true of a cell 

 undergoing transformation), and can be called 

 a merozygote, produced by a process of par- 

 tial genetic exchange or meromixis. The last 

 two new terms are applicable also in trans- 

 formation. 



While the frequency of recombination is 

 low for all chromosomal genes in an Lfr 

 strain, it is relatively very high for markers 

 in Hfr nearest O, decreases as the distance 

 of the markers from O increases, and is only 

 .01 -.001% for the markers most distal to O. 

 It was mentioned that only rarely is an off- 

 spring of Hfr X F" itself Hfr. This happens 

 only in cases where there is evidence that the 

 marker most distal to O has also been trans- 

 ferred. This suggests that the locus re- 

 sponsible for Hfr is located on the chromo- 

 some. Moreover, no chromosomal locus is 

 found to be transferred after the Hfr locus. 

 We conclude, therefore, that Hfr is always 

 located at the terminus of the chromosome 

 which is in the process of being transferred. 



A fourth mating type is known,'* derived 

 from F+ cultures, which produces a very 

 //igh /requency of recombination, and is 

 appropriately called Vhf. In Vhf strains (all 

 are male), recombination rates for the mark- 

 ers most distal to O occur with a frequency 

 of 1-2%, a rate at least 100 times that 

 found in Hfr strains. (Even so, less than 

 about 1% of the progeny from mating Vhf 

 and F are Vhf.) Three independently arisen 

 Vhf strains are known. By means of artificial 

 rupture experiments, the sequence of certain 

 marker genes can be determined in each case. 

 These sequences are shown in Figure 39-3, 



^ See A. L. Taylor and E. A. Adelberg (1960). 



together with the frequency of recombinants 

 per 100 Vhf cells in the mating mixture. 

 What do these results show? 



The different Vhf strains demonstrate that 

 the markers held in common are in the same 

 sequence. However, the O point is in a differ- 

 ent position in each case! Accordingly, so 

 is the position of the Vhf locus at the end of 

 the linkage map. This suggests the following 

 hypotheses.^ The linkage group of E. coli 

 is normally circular; the location of the Vhf- 

 causing factor can occasionally change; be- 

 fore chromosome transfer the linkage group 

 is opened adjacent to the point of Vhf attach- 

 ment, so that the Vhf locus is at the end oppo- 

 ^ Following F. Jacob and E, L. Wollman. 



FIGURE 39-3. Recombination percentages for Vhf 

 strains. O = point of origin; — = untested. 

 A. Adelberg, 1960. 



{After A. L. Taylor and E. 

 See References.) 



SELECTED 

 MARKER 



his 



-I- 

 gal 



+ 

 pro 



+ 

 met 



+ 

 mtl 



xyl 



+ 

 mal 



ade 



+ 



org 



AB-3n 



42 

 12 



STRAIN 

 AB-312 



2.5 



4 



8 



AB-313 



