332 



( II \I»TER 25 



In other words, the phage is passive with 

 respect to the content of genes it transduces 

 and retains no transducing memoiy oi an\ 

 hosts previous to the last. Since additional 

 tests demonstrate that ever) locus in Sal- 

 monella is transduceable by P22, we can 

 call this a case of unrestricted or general- 

 ized transduction. In generalized transduc- 

 tion one cell is transduced for a given 

 marker for about each 10''' infecting phage 

 particles. 



Any chromosomal marker is transduce- 

 able by P22. but is it possible to transduce 

 more than one at a time? P22 can be 

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

 grown on M~T~X-. The latter bacteria 

 are replica-plated on three different media 



one selecting only for M+ recombinants 



(it contains T and X), another only for 

 T+, and the third only for X l . When the 

 M+ clones are further typed, they are still 

 T-X~. Similarly. r+ clones are still 

 M-X~, and X+ clones are still M~T~. 

 These results show that only a single bac- 

 terial marker or a relatively short DNA seg- 

 ment is transduced at one time. In this re- 

 spect, transduction is similar to transforma- 

 tion but different from conjugation, in which 

 — especially in Hfr strains — large sequences 

 of genes can be transmitted and integrated. 

 In Salmonella, however, examples are 

 known of several genetic markers trans- 

 duced together in what is called linked trans- 

 duction or cotransduction. Other work has 

 established that the biological synthesis of 

 the amino acid, tryptophan, is part of a se- 

 quence of genetically-determined reactions 

 that proceed from anthranilic acid through 

 indole to tryptophan. Different genes con- 

 trolling different steps of this biosynthetic 

 sequence are cotransduced ■; this finding 

 suggests such genes are closely linked to 

 each other. The biosynthesis of histidine in 

 Salmonella is known to involve at least eight 

 loci, four of which produce identifiable ef- 

 • As shown by M. Demerec and coworkers. 



fects on the sequence of chemical reactions 

 involved. Linked transductions have been 

 found between two or more of these loci. 4 

 In fact, using the relative frequencies of dif- 

 ferent cotransductions and other evidence. 

 all eight loci are found to be continuous with 

 each other and to lie arranged linearly (see 

 Figure 25-1). Using cotransduction, one 

 can build up a complete and detailed ge- 

 netic map of Salmonella which proves to be 

 a single circle. Cotransduction of closely- 

 linked markers is also known to occur " in 

 E. coli by phage P 1 . 



In a generalized transduction experiment, 

 when a prototroph is obtained by transduc- 

 ing an auxotroph, the new prototroph is 

 usually stable and produces clones pheno- 

 typically identical to typical prototrophs. 

 This process is called complete transduc- 

 tion. In this case, the prototrophic gene 

 introduced must have integrated into the 

 Salmonella chromosome in place of the re- 

 cipient's auxotrophic allele. However, in 

 addition to the large prototrophic colonies 

 formed on selective agar (each of these 

 clones represents a complete transduction), 

 on occasion about ten times as many minute 

 colonies are present (see Figure 25-2). 

 These minute colonies do not appear in 

 platings of auxotrophic mutants on minimal 

 medium and, also, are not the result of an 

 interaction between auxotrophs and colonies 

 of normal or transduced prototrophs located 

 elsewhere on the plate. Minute colony for- 

 mation is explained as follows: Through 

 phage infection the cell initiating the minute 

 colony receives the segment of DNA con- 

 taining the gene for prototrophy under test. 

 This gene, however, fails to be integrated 

 and fails to replicate but retains its func- 

 tional ability to produce a phenotypic effect. 

 Consequently, a hybrid merogenote or het- 

 erogenote is produced in which the domi- 

 nant injected gene for prototrophy is func- 



4 By M. Demerec. P. E. Hartman. and coworkers. 

 ■ From the work of E. Lennox. 



