392 



CHAPTER 43 



Prophage has another characteristic, 

 namely, the capacity to produce infective 

 phage. It should be noted that while the 

 chromosomal locus and immunity properties 

 of prophage are explicable on the basis of a 

 small portion of the phage's total DNA 

 content, the Ci region, this does not neces- 

 sarily mean that this region alone includes all 

 the genetic information or specifications 

 needed for prophage to become vegetative 

 phage, and therefore infective phage. 



The preceding discussion may suggest to 

 you that not all of the genetic material present 

 in a phage is essential for the existence of 

 phage as an organism. This possibility 

 should not be surprising, since we are already 

 acquainted, in other organisms, with homo- 

 zygous deficiencies (of euchromatic or hetero- 

 chromatic regions) which still permit viability 

 of the organism (even though such a change 

 may be more or less adaptive). In these 

 cases, then, there is genetic material present 

 which is nonessential for the formation of the 

 organism. Although we have seen that in 

 order to have a phage organism at least the 

 prophage loci in the Ci region are essential, 

 we do not have any way, at present, of 

 determining how many additional loci are 

 essential. 



Let us assume, contrary to the expectation 

 just mentioned, that all genes present in 

 phage are essential for phage existence. If 

 so, one would expect to find that progeny 

 phage contain the same genetic material as 

 the parent phage. Or, to put this expectation 

 another way, the very DNA molecules pres- 

 ent in the parental phage particle should be 

 present in one or more progeny phage. This 

 can be tested in the following way. The DNA 

 of phage is labeled with P^-', by harvesting the 

 phage that lyse bacteria growing in medium 

 whose sole P source is P^'-. The labeled phages 

 are then permitted to infect unlabeled bac- 

 teria, and the total amount of label included 

 in the harvested progeny is compared with 

 the total amount present in the parent phage. 



It is found ^ that only about 40% of the 

 parentally labeled DNA is included in the 

 progeny, that is, 60% of the specific DNA 

 nucleotides (P atoms) present in the parents 

 are not found in the offspring. Thus, 60% 

 of the DNA of phage is nonconserved. There 

 are several explanations possible for this. 

 One possibility is that all or almost all of the 

 genetic material of phage is essential but that 

 replication and the formation of progeny 

 phage is inefficient, so that not all the parental 

 DNA is retained, 60% of it being replaced by 

 unlabeled daughter DNA genetic material of 

 exactly the same type. The presence of a 

 mixture of labeled and unlabeled DNA in the 

 same particle indicates the occurrence of 

 genetic recombination (see p. 388). Even 

 though the parental phage DNA is sometimes 

 broken to produce genetic recombinant prog- 

 eny, there is very clear evidence^ that the 

 DNA of phage T2 normally consists of a 

 single unbroken molecule whose molecular 

 weight is 130-160 X 10«. 



In discussing the advantages of bacteria as 

 material for genetic studies (p. 330), it was 

 noted that very large bacterial populations 

 are readily manipulated experimentally. This 

 makes it feasible to discover and study rare 

 events of mutation and genetic recombination. 

 Phage also has this particular advantage. 

 Consider how this advantage may be put to 

 use in the investigation of a particular kind 

 of phage mutant,"* whose study may reveal 

 the characteristics of the fine structure of the 

 genetic material. 



We have already mentioned (on p. 387) 

 that wild -type (r+) T-even phages produce 

 small, rough-edged plaques when plated on 

 E. coli, whereas rapid lysis (r) mutants pro- 



2 By J. D. Watson and O. Maal0e, by C. Levinthal, 

 and by others. 



3 From I. Rubenstein, C. A. Thomas, Jr., and A. D. 

 Hershey (1961), and P. F. Davidson, D. Freifelder, 

 R. Hede, and C. Levinthal (1961). 



"• The discussion following is based largely upon the 

 work of S. Benzer (1955, 1957). 



