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CHAPTER 37 



high as 25%. Such results demonstrate that 

 transformation is not rare. In view of the 

 possibility that transformation may be a 

 routine process, it is best not to consider it 

 as mutation. Accordingly, transformation, 

 like segregation, independent segregation, 

 crossing over, and fertilization, is probably 

 best considered as a potential mechanism for 

 normal genetic recombination. 



It is now believed that completion of the 

 transformation process requires a series of 

 discrete stages. 



1. Cell competence. There are certain 

 periods in cell division or in the growth of a 

 culture during which transformation does not 

 occur, whereas in other periods the cells are 

 competent to react. 



2. Binding of the transformer. When cells 

 are in a competent stage, the transforming 

 DNA is first transiently bound to the cell, 

 and can be removed by several methods, in- 

 cluding the action of added DNAase, before 

 it is bound permanently. 



3. Penetration of transformer. Perma- 

 nently bound DNA is considered to have 

 penetrated the recipient cell. If the trans- 

 former DNA has been fragmented sonically, 

 so that the DNA particles have a molecular 

 weight of less than 4 X 10% these do not 

 penetrate. Only high molecular weight DNA 

 penetrates. Pneiimococcus contains about 6 

 million pairs of nucleotides per haploid nu- 

 cleus, equivalent to about 200 molecules of 

 about 10 X 10'' molecular weight. It has 

 been found that about 60-4800 molecules of 

 such a molecular weight penetrate. It is 

 clear, therefore, that a large amount of DNA 

 (equal to a considerable portion of the donor 

 genome) succeeds in penetration. This, plus 

 the fact that smaller pieces of DNA cannot 

 penetrate, recalls the circumstances under 

 which DNA uptake occurs in tissue culture 

 (see p. 317). In such cases, DNA enters by 

 phagocytosis, which occurs only when the 

 DNA adheres to a suitably large particle. 

 Perhaps, in bacteria also, a mechanism of 



penetration is involved in which the cell also 

 takes an active part, and can do so only if 

 the amount of DNA is sufficiently large. (It 

 may be noted that the success of transforma- 

 tion is inversely related to the thickness of a 

 polysaccharide coat, which probably acts as 

 some kind of barrier to binding or penetra- 

 tion.) 



Whatever the specific mechanism for pene- 

 tration may be, it is known that there are a 

 finite number of sites on the bacterial surface 

 which act as receptors for DNA. Since non- 

 transforming DNA, such as DNA from a 

 widely separated genus, can also penetrate 

 readily, receptor sites can be saturated by 

 nontransforming DNA, after which trans- 

 forming DNA cannot penetrate. 



4. Synapsis. Alternatives of the same trait 

 (for example, resistance and sensitivity to 

 streptomycin, or auxotrophy and prototro- 

 phy for a particular nutrient) can be found in 

 diff'erent species of bacteria. On the reason- 

 able assumption that the same gene and its 

 alternatives perform the same functions in 

 different species, it ought to be possible to 

 produce interspecific transformations. This 

 has been accomplished, but, in any given 

 case, interspecific transformation is usually 

 less frequent than the intraspecific one. The 

 fact that interspecific transformation takes 

 place at all favors the idea that the trans- 

 formed locus is normally part of the genotype 

 of both species. The scarcity of interspecific 

 transformations, therefore, is probably not 

 due to the locus transformed; nor is it due 

 to any failure of competence, or of binding 

 or penetration of the foreign DNA. More- 

 over, the transformation rate is actually lower, 

 and is not an artifact due to a delay in pheno- 

 typic expression which might occur in inter- 

 specific, but not in intraspecific, transforma- 

 tion. 



We are led, therefore, to hypothesize that 

 the capacity to transform, which already- 

 penetrated DNA has, is dependent upon the 

 nature of the genes adjacent to those whose 



