RECOMBINATION IN VIRUSES AND BACTERIA 



129 



and acts as a template for the replication that next occurs. Clearly, 

 such a copy-choice mechanism will result in the substitution of one gene 

 for another in the newly formed DNA. If two mutants differ in the site 

 that has mutated, one can transduce the other; if the mutations are at 

 the same site, a wild type cannot result from transduction (Figure 5.9). 

 Thus a series of gal' mutants can be arranged into groups according to 

 whether one can transduce the other. When transduction does not 

 occur, it is likely that the two mutants have lesions at the same or 

 closely linked sites; when transduction does occur, we can conclude with 

 assurance that the sites are different. There seems to be some preference 

 in the sequence of nucleotides which will be copied, for it appears 

 that the wild-type allele may more easily replace its opposite in trans- 

 duction than vice versa. This model accounts for the incorporation of 

 the information carried in the transduced DNA into the replicating gen- 

 etic material of the recipient, and so it should, for transduced material 

 usually multiplies in .synchrony with the rest of the genotype. 



Sometimes, however, the transduced bit of DNA takes up residence in 

 the new host cell but does not multiply at all. It controls the phenotype 

 of the cell but, upon division, is passed to one or the other daughter. 

 Such a situation is referred to as abortive transduction, and the cells that 

 contain the transduced DNA in series are called a line (in contrast to a 

 clone, which expands in number). A striking example of abortive trans- 

 duction involves flagella formation, and hence motility, in Salmonella. 



When a flagellaless culture is treated with phage from a motile one, 

 some of its members are transduced to motility. Stable transductants 

 divide and swim away from the parent colony, forming extensions from 

 it (Figure 5.10). Abortive transductants also swim away but, upon divi- 



Stable 

 transduction 



/ \ 



/ \ / \ 



Transduced clone 

 of motile cells 



:cz3 



:CD. 

 :cz)- 



:cD 



"CD 



Clones of 

 nonmotile cells 



Transduced line 

 of motile cells 



FIGURE 5.10. Two types of transduction in Salmonella, stable and abortive. Non- 

 motile bacteria mixed with phages from a motile strain form a parent colony from which 

 transductants emerge (after Stocker, 1956, J. Gen. Microbiol., 15:575). 



