340 



CHAPTER 26 



oi all viruses. Certain plant viruses, such 

 is the tobacco mosaic and turnip yellow 

 mosaic, arc relative!) simple helical or 

 spherical structures. Although </>X174 

 seems to have a simple spherical structure. 

 4>R, a closely related single-stranded DNA 

 phage, shows a small knob which may func- 

 tion as a tail. 



Identification of the genetic material in 

 DNA-containing phages is made somewhat 

 easier because DNA contains no sulphur 

 and T2 phage protein contains no phospho- 

 rous. The DNA in one sample of phage 

 can be labeled by feeding the E. coli host 

 cells radioactive P : -, while the protein in 

 another phage sample is labeled by feeding 

 the host cells radioactive S 35 . Each sample 

 of radioactive phage is then permitted to 

 infect nonlabeled cells. '■ The following re- 

 sults are obtained: in one sample all of the 

 P 32 (hence all of the DNA) enters the bac- 

 terium; in the other all but about 3% of 

 the S 35 (hence almost all the protein) re- 

 mains outside and is removed by blendor 

 treatment. As implied earlier (p. 330), 

 when most of the protein of an attached 

 phage is removed from the host cell by 

 blendor treatment the normal outcome of 

 infection remains unaffected. These results 

 are consistent with the view that DNA and 

 not protein is the carrier of phage genetic 

 information. 



; This account follows the work of A. D. Hershey 

 and M. Chase ( 1952). 



Recall (p. 336) that pure DNA does not 

 penetrate normal E. coli unassisted. The 

 cell wall of /.. coli can be removed by suit- 

 able culture conditions, leaving a protoplast 

 that can be penetrated by purified DNA. 

 After phenol, CaCT, or other treatments, 

 the entire protein coat of phage can be re- 

 moved, leaving pure DNA. When proto- 

 plasts of /•-'. coli are mixed with such pure, 

 single-stranded DNA of phage XI 74 (which 

 has only about 4,500 dcoxyribotides per par- 

 ticle), typical <£X174 progeny, including the 

 characteristic protein envelope arc pro- 

 duced. 1 Consequently, the only genetic ma- 

 terial in DNA-containing phage is DNA. 



The course of events leading to lysis of 

 a phage-infected bacterium can be sum- 

 marized as follows (Figure 26-2): The 

 phage becomes attached tail-first to specific 

 receptors on the bacterial surface. All the 

 DNA and a small amount of protein are 

 injected into the host; probably, the injec- 

 tion is assisted by the contraction of the 

 spiral sheath protein. An eclipse period 

 follows (Figure 26-2, B-D), during which 

 no infective phage can be recovered if the 

 host cell is artificially lysed. During this 

 eclipse period, the infected cell is said to 

 carry immature phage, and the phage DNA 

 is replicating to produce a pool of phage 

 DNA units. Starting at the end of the 

 eclipse period (Figure 26-2E), a fraction 



1 This has been shown by G. D. Guthrie and R. L. 

 Sinsheimer. 



FIGURE 26-2 (opposite) . Electron micrographs of growth of T2 virus inside the E. coli 

 host cell. A. Bacillus before infection. B. Four minutes after infection. C. Ten min- 

 utes after infection. The thin section photographed includes the protein coat of T2 

 which can he seen attached to the bacterial surface. D. Twelve minutes after infection. 

 New virus particles are starting to condense. E. Thirty minutes after infection. More 

 than 50 T2 particles are completely formed and the host is about ready to lyse. (Cour- 

 tesy of E. Kellenberger. Reprinted from the Scientific American, 204:100, 196E) 



