298 



( II M'TI.R 22 



particles have a molecular weight of less 

 than 4 times H>\ which is not sufficient 



to penetrate. Only high-molecular-weight 

 DNA penetrates. 



These tacts should be considered with re- 

 gard to the circumstances under which DNA 

 uptake occurs in mammalian tissue culture. 

 In this case, DNA enters by phagocytosis 

 which occurs only when the DNA adheres 

 to a suitably large non-DNA particle. Pino- 

 cytosis, similar to phagocytosis, is another 

 process by which materials can enter ordi- 

 nary cells. Although pure nucleic acids are 

 not pinocytosed, protein is. However, if 

 pure nucleic acid is mixed with protein, 

 pinocytosis is stimulated, and the nucleic 

 acid is carried into the cell with the protein. 

 Perhaps the penetration of high-molecular- 

 weight DNA into bacteria is dependent upon 

 the presence of sufficient contaminating ma- 

 terial capable of stimulating pinocytosis or 

 some other mechanism for DNA penetra- 

 tion. Whatever the precise method by which 

 transforming DNA penetrates, it is found 

 that relatively short sequences of DNA will 

 enter microbial cells if sufficient protein is 

 also present. 



The bacterial surface contains a finite 

 number of sites 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, thereby 

 preventing the penetration of transforming 

 DNA. 



Synapsis. Alternatives of the same trait 

 — for example, resistance and sensitivity to 

 streptomycin, or auxotrophy and prototrophy 

 for a particular nutrient — can be found in 

 different species of bacteria. Since it is a 

 reasonable assumption that the same type 

 i)| gene (and its alternatives) performs the 

 same or similar functions in different species, 

 interspecific transformations ought to be 

 possible. Although this result has been 

 achieved, in any given case the interspecific 



transformation is usually less frequent than 

 the intraspecific one. Moreover, the trans- 

 formation frequency is actually lower and 

 not due to a delay in phenotypic expression 

 which occurs in interspecific (but not in 

 intraspecific) transformation. That inter- 

 specific transformation does take place favors 

 the idea that the transformed locus is nor- 

 mally part of the genotype of both species. 

 The relative infrequency of interspecific 

 transformations is, therefore, not due to in- 

 competence of the recipient cell or a failure 

 of the foreign DNA to bind to or penetrate 

 the recipient. 



The transforming capacity of already- 

 penetrated DNA may depend not only upon 

 the homology of the loci transformed but 

 upon the nature of the genes adjacent to those 

 undergoing transformation. These neigh- 

 boring genes might influence transformation 

 by their effect upon the synapsis of the trans- 

 forming DNA with the corresponding region 

 of the host's genetic material. In intraspe- 

 cific transformation, the loci adjacent to 

 those transformed are very probably homol- 

 ogous in transformer and host, so that syn- 

 apsis between the two segments can occur 

 properly; in interspecific transformation, 

 these loci are likely to be nonhomologous 

 and, therefore, may often fail to synapse or 

 act to prevent synapsis. 



Integration. Even if the hypothesized 

 synapsis occurs properly between host and 

 transforming DNA, some process has yet 

 to occur by which the host gene — whose 

 transformation is being followed — is lost 

 from the chromosome, and the donor's locus 

 becomes an integral part of it. Some under- 

 standing of the mechanism of this final stage 

 in transformation may be gained from a 

 study of transformation frequency. First of 

 all. different loci transform intraspecifically 

 with different frequencies. Using genes that 

 transform with suitably high frequencies, we 

 are able to study the frequency of double 

 transformations, that is, the frequency with 



