360 



CHAPTER 39 



site the O point. The results obtained from 

 the study of Hfr strains ^ confirm all these 

 assumptions, including the occasional reloca- 

 tion of Hfr as a consequence of which a new 

 O point and sequence of entry is determined. 

 To what can we attribute the difference be- 

 tween Vhf and Hfr strains? One simple 

 explanation is that these differ in the rate 

 with which they cause spontaneous chromo- 

 some rupture during conjugation, Vhf having 

 the lower rate. 



There is a remarkable similarity between a 

 Vhf or Hfr locus in the chromosome and its 

 capacity to produce breakages in nearby 

 regions, on the one hand, and certain cases 

 already described on pp. 214-222 in corn 

 (Activator and Dissociation, and Modulator), 

 or referred to on pp. 222 and 223 in Drosophila 

 (Segregation-Distorter), on the other hand. 

 Suffice it to say, at this point, that these cases 

 may provide another example of what at first 

 appears to be a wide variety of apparently 

 different and unique phenomena and which 

 later proves to be due to minor variations in 

 the expression of a more general, common 

 event. In the light of results with higher 

 organisms, it is reasonable that the positions 

 of spontaneous rupture are merely places 

 where the already opened E. co// chromosome 

 is subsequently broken by the action of Hfr, 

 and to a lesser degree Vhf, these breakages 

 being more probable the closer the region is 

 to the Hfr or Vhf locus. It may be noted 

 that the results are not contrary to the view 

 that once the chromosome is broken, Hfr (or 

 Vhf) ceases to cause additional breakages in 

 the now-free, other segment. 



Since a frequently recombining male strain 

 always has the same marker leading the 

 others in transfer, we conclude that Hfr or 

 Vhf can cause the ring chromosome to open 

 at only one of the two regions immediately 

 adjacent to it. The fact that the entry se- 

 quence is different in different strains (the 

 chromosome of AB-312 enters in the reverse 

 * See A. L. Taylor and E. A. Adelberg (1961). 



direction from that of AB-311 or AB-313, 

 as can be seen from Figures 39-3 and 39-4) 

 may be due to an inversion of Hfr or of Vhf 

 in moving from one chromosomal position 

 to another. 



There is experimental evidence that the 

 E. coli chromosome is composed of a single 

 double-stranded DNA helix, although it is 

 possible that there may be nonnucleic acid 

 links between DNA molecules (each with a 

 molecular weight of about 10 X 10^ and 

 about 16,000 nucleotides long). (The ordi- 

 nary chromosome of higher organisms may 

 be composed of one or of as many as 16 

 double strands of DNA ; we do not yet know 

 for certain how many DNA strands are pres- 

 ent per chromatid.) Since the ring chromo- 

 some of E. coli is opened where F attaches, 

 you may wonder if these two new ends are 

 able to join in restitution. These two open 

 ends must usually remain unjoined, other- 

 wise one would not observe, in Vhf lines, the 

 selective marker nearest O transported and 

 integrated from as many as 49% of donor 

 males (Figure 39-3). 



The recombination frequencies observed 

 after conjugation will depend, of course, upon 

 the frequency of penetration of a marker plus 

 the efficiency with which it becomes inte- 

 grated. Interruption of mating experiments 

 reveal the sequence of markers, regardless of 

 the frequency (greater than 0) with which 

 their integration occurs. Once the marker 

 sequence is known, integration efficiency can 

 be studied. For example, if matings are 

 permitted to continue long enough so that 

 just about all F~ cells are penetrated by the 

 marker closest to O, the percentage of zygotes 

 producing recombinants for that marker will 

 indicate the efficiency of integration. Thus, 

 if only about 20% of the recipient cells show 

 integration of a marker near O, this locus 

 has an integration efficiency of %. By essen- 

 tially similar methods the integration effi- 

 ciency for markers more distal to O can be 

 determined. These are found to be of lower 



