380 



CHAPTER 41 



infecting phage contents do, in accord with 

 the expectation mentioned on page 374. 

 Indeed, this is known to occur; ^'^ that is, using 

 nontransducing lambda as a carrier or "help- 

 er," DNA isolated from defective transduc- 

 ing phage is capable of Gal transformation. 



You should have noticed, in the discussion 

 of lambda, that such a temperate phage has 

 two alternative pathways of action open to it 

 upon infecting a sensitive bacterium. Either 

 it enters the cytoplasm where it replicates 

 faster than the chromosome, or it integrates 

 with the chromosome where, as prophage, it 

 resides associated with the Lp locus and is 

 synchronously replicated as a regular chro- 

 mosomal marker. Accordingly, lambda, and 

 probably most, if not all, other temperate 

 phages are episomes. 



What is the difference between temperate 

 phages capable of generalized and restricted 



^" From the work of A. D. Kaiser and D. S. Hogness 

 (1960). 



transduction? Extrapolating our reasoning 

 regarding the episome F, it can be suggested 

 that the nucleotide sequence held in common 

 between prophage and chromosome is shorter 

 for generalized transducing phages than it is 

 for restrictive transducing phages, the former 

 having a greater number of, and hence less 

 specific, places of attachment as prophage 

 than do the latter. Several experiments sup- 

 port the view that, at least sometimes, pro- 

 phage may make the host cell immune to 

 further infection by homologous phage, not 

 by preventing the penetration of the DNA, 

 but by preventing its replication. This action 

 parallels the suppression of free-F replication 

 by integrated F. 



Transduction by temperate phages has been 

 found to occur also in Pseudomonas, Vibrio, 

 Staphylococcus, and Proteus. It would not 

 be surprising to find that transduction can 

 occur in a wide variety of cells, including 

 human cells. 



SUMMARY AND CONCLUSIONS 



Genetic recombination of loci characteristically identified with the bacterial chromosome 

 can be mediated by temperate bacteriophages in the processes of genetic transduction and 

 genetic transformation. 



A transducing phage is defective in its own genome. The deficient portion is probably 

 replaced by a small segment of DNA acquired at the time the prophage deintegrated from 

 its last host. The transduced segment may be derived from a wide variety of regions of the 

 bacterial chromosome, as in generalized transduction, or from a narrowly limited region, 

 as in restricted transduction. The DNA segment transduced may, by integration, replace a 

 chromosomal marker of the host (as in complete transduction), or it may produce a merozy- 

 gote, in which case the exogenote is still capable of acting cistronically, whether it can 

 replicate (as can Gal exogenotes in E. coli) or cannot (as in abortive transduction in Sal- 

 monella). 



Most, if not all, temperate phages are episomes, which when attached to the chromosome 

 have some characteristics resembling those of attached F. 



REFERENCES 



Arber, W., Kellenberger, G., and Weigle, J., "The Defectiveness of Lambda-Transducing 

 Phage" (in French), Schweiz. Zeitschr. Allgemeine Path, und Bact., 20:659-665, 

 1957; translated and reprinted in Papers on Bacterial Genetics, Adelberg, E. A. 

 (Ed.), Boston, Little, Brown, 1960, pp. 224-229. 



Jacob, F., and WoUman, E. L., "Spontaneous Induction of the Development of Bacterio- 

 phage X in Genetic Recombination of Escherichia Coli K12" (in French), C. R. Acad. 

 Sci., Paris, 239:317-319, 1954; translated and reprinted in Papers on Bacterial 

 Viruses, Stent, G. S. (Ed.), Boston, Little, Brown, 1960, pp. 336-338. 



