376 



CHAPTER 41 



but not of Y+ or Z+. Another part of the 

 harvested phage is not tested yet but grown in- 

 stead on another genetically marked strain, 

 for example, M+TX^ Y+Z . The new crop of 

 phage produced is harvested and then tested 

 on the indicator strains already mentioned. 

 It is found now that the new crop has lost T+ 

 transducing ability but has gained Y+ trans- 

 ducing capacity. These results demonstrate 

 that a phage filtrate has a range of markers 

 for transduction which is exactly the range of 

 the markers present in the bacteria on which 

 the phage was last grown. In other words, 

 the phage is passive with respect to the con- 

 tent of genes it transduces, and retains no 

 transducing memory from hosts previous to 

 the last. Since additional tests demonstrate 

 that practically any locus in Salmonella is 

 transduceable by P22, we may call this a case 

 of generalized transduction. In generalized 

 transduction a particular marker is trans- 

 duced once for about each lO*' infecting phage 

 particles. 



Although almost any chromosomal marker 

 is transduceable by P22, what is the length or 

 scope of the transduced DNA? P22 can be 

 grown on M+T+X+, harvested, and then grown 

 on M-J-X-. The latter bacteria are replica- 

 plated on different media, of which one 

 selects for A/+ recombinants, another selects 

 for r+, and a third selects for X+. When the 

 M+ clones are further typed they are still 

 T-X-. Similarly, T+ clones are still M X^ 

 and X+ clones are still M~T~. This demon- 

 strates that usually a single bacterial marker 

 is transduced. In this respect, then, trans- 

 duction is similar to transformation but is 

 different from conjugation, where, especially 

 in Vhf strains, large blocks of genes may be 

 transmitted and integrated. 



Several examples are known in Salmonella, 

 however, in which several genetic markers are 

 transduced together, in what may be called 

 linked transduction or cotransduction. It was 

 established, in other work, that the biological 

 synthesis of the amino acid tryptophan is 



part of a sequence of genetically determined 

 reactions that proceeds from anthranilic acid 

 to indol to tryptophan. Cotransductions 

 were found ^ of genes controlling different 

 steps of this biosynthetic sequence, indicating 

 that these genes are closely linked to each 

 other. 



Histidine biosynthesis in Salmonella in- 

 volves at least eight loci, of which four pro- 

 duce identifiable effects on the sequence of 

 chemical reactions involved. Linked trans- 

 ductions have been found between two or 

 more of these loci."* In fact, using the relative 

 frequencies of different cotransductions and 

 other evidences, it was possible to prove that 

 all eight loci are continuous with each other 

 and are arranged linearly (see Figure 46-1, 

 page 422). The close linkage of cistrons 

 controUing different parts of a biosynthetic 

 sequence is not a universal phenomenon, 

 however. But, when such close linkage does 

 occur, it may be adaptive, in that a single 

 mechanism may suffice to turn off or on the 

 whole series of enzymatic reactions. (In this 

 regard it may be suggested that nucleotide- 

 sharing could provide a mechanism for turn- 

 ing chemical sequences on or off, depending 

 upon which of the overlapping cistrons was 

 functional on different occasions.) Cotrans- 

 duction of closely linked markers is known to 

 occur ^ also in E. coli infected with phage PI. 



When, in a generalized transduction experi- 

 ment, a prototroph is obtained by transducing 

 an auxotroph, the new prototroph is stable 

 and produces clones phenotypically identical 

 to typical prototrophs. We may call this 

 complete transduction. In this case, the intro- 

 duced prototrophic gene must have inte- 

 grated into the Salmonella chromosome in 

 place of the recipient's auxotrophic gene. It 

 was noticed, however, that in addition to the 

 large colonies formed, each of which repre- 

 sented a complete transduction, there were 



^ By M. Demerec and coworkers. 



'' By M. Demerec, P. E. Hartman, and coworkers. 



^ From the work of E. Lennox. 



