Bacteria : Recombination (I) 



343 



transformation is under test. These neighbor 

 genes can be visualized as influencing trans- 

 formation by their effect upon synapsis be- 

 tween the transforming DNA and the cor- 

 responding region of the host's genetic ma- 

 terial. In intraspecific transformation, the 

 loci adjacent to the transformed ones are 

 homologous in transformer and host, so that 

 synapsis between the two segments may occur 

 properly, whereas in interspecific transforma- 

 tion, these loci are likely to be nonhomolo- 

 gous (or act to prevent synapsis), and, there- 

 fore, may often fail to synapse. 



5. Integration. Even if the hypothesized 

 synapsis occurs properly between host and 

 transformer DNA, some process has yet to 

 occur by which the host gene, whose trans- 

 formation is being followed, is lost from the 

 chromosome, and the new locus becomes an 

 integral part of it. Some understanding of 

 the mechanism of this final stage in trans- 

 formation may be gained from a study of the 

 frequency of transformation. Diff'erent loci 

 transform intraspecifically at diff'erent rates. 

 Using genes that transform with suitably high 

 frequencies, it has been possible to study the 

 rate of double transformations, that is, the 

 frequency with which bacteria are trans- 

 formed with respect to two markers present 

 in the donor DNA. In several cases (for 

 example, penicillin- and streptomycin-re- 

 sistance), the frequency of doubly trans- 

 formed bacteria is somewhat less than the 

 product of the frequencies for the single 

 transformations. In these cases, this means 

 that the transformer DNA carries the two 

 loci either on separate particles, or else in 

 widely separated positions on the same 

 particle. On the other hand, the markers for 

 streptomycin-resistance and mannitol-fer- 

 mentation are transformed together with a 

 frequency (.1%) which is about 17 times that 

 expected from the product of the frequencies 

 of the single transformations (.006%). This 

 is taken to mean that these two genetic mark- 

 ers are located on the same transforming 



particle, i.e., they are reasonably close to- 

 gether in the same bacterial chromosome. 



How can we explain single and double 

 transformations of closely linked loci? Be- 

 cause of fragmentation during extraction, a 

 given penetrating DNA particle may not 

 always have the same composition relative 

 to the two markers; it may sometimes carry 

 only one of these, and at other times carry 

 both markers. We can test the eff'ect, on 

 single and double transformations, of reduc- 

 ing the particle size of penetrating DNA. 

 On the present hypothesis, we would expect, 

 when particle size is reduced by DNAase 

 or sonic treatment, that sometimes the par- 

 ticles would be broken between the two 

 markers, thereby reducing the relative fre- 

 quency of the double transformation and 

 increasing the relative frequencies of the single 

 transformations. When the particle size is 

 reduced, the over-all rate of transformation 

 is lower, as expected. However, no change 

 is found in the ratio of double to single trans- 

 formations. This must mean that the two 

 markers are so closely linked that they are 

 separated only rarely when particles are frag- 

 mented. Accordingly, this may be taken to 

 mean that the penetrating particles must 

 usually carry both markers, or neither, and 

 that the failure to obtain 100% double trans- 

 formations from the former type must be due 

 to the fact that only a small portion of such 

 a penetrant, synapsing particle is integrated. 



Integration of a portion of a synapsed 

 particle may be visualized as occurring in 

 two possible ways (Figure 37-1). One method 

 could involve "breakage and exchange" of 

 the kind that takes place in chromosomal 

 rearrangement or crossing over. In this case 

 (Figure 37-1 A), "breaks" would have to 

 occur on each side of the marker to be intro- 

 duced, so that a "2-strand-double crossover" 

 (refer to p. 133) is produced. While double 

 crossovers within a short distance would be 

 expected to be extremely rare between two 

 homologous chromosomes of higher organ- 



