Chapter 26 



BACTERIOPHAGE: RECOMBINATION 

 AND GENETIC MAPS 



Tl 



I he morphology of the T-even 

 group (T2, T4, T6) of phages 

 that attack E. coli has been 

 studied in some detail. 1 Its members are 

 tadpole shaped, 0.1 to 0.2 p. long — roughly 

 a tenth the bacterial diameter (Figure 26- 

 1 ) . The surface of the head has a hexa- 

 gonal outline and facets like a crystal. The 

 head membrane is composed of numerous 

 subunits each having a molecular weight of 

 about 80,000. The tail is cylindrical and 

 is used by the phage for attachment to the 

 host cell. The outer sheath of the tail, com- 

 posed of about 200 spirally-arranged sub- 

 units each having a molecular weight of 

 approximately 50,000, forms a hollow cylin- 

 der. The sheath can contract, shortening 

 its length while increasing its diameter with- 

 out changing its volume appreciably. Be- 

 neath the sheath is the core, a hollow cyl- 

 inder with a central hole about 25A in 

 diameter. At the distal end of the core is a 

 hexagonal plate to which six tail fibers are 

 attached; each is bent in the middle and 

 seems to contain subunits with molecular 

 weight of not less than 100,000 a piece. 

 The subunits of the head membrane, sheath, 

 and tail fibers are composed of protein. 

 When digested with trypsin, each of these 

 subunits produces a unique set of peptides 

 indicating that each is different. The core 

 is also protein. A serologically distinct pro- 

 tein, 4 to 6% of the total phage protein, 

 is found in the interior of the phage par- 



1 See S. Brenner et al. (1959). 

 339 



tide; polyamines, putrescine, spermadine, 

 lysozymc. and a minor polypeptide are also 

 reported in the phage interior. 



In addition to these components, the T- 

 even phage interior contains DNA whose 

 volume is approximately the same as that 

 of the total protein. This DNA is com- 

 posed of a single double helix about 200,- 

 000 nucleotides long. Since such a poly- 

 nucleotide would be about 68 p. long, the 

 DNA inside the phage must be highly 

 coiled.- No RNA has been reported in 

 DNA-containing phages. 



Not all phages contain DNA; several bac- 

 teriophage contain RNA and no DNA. 

 Moreover, the physical and chemical com- 

 plexity of the T-even phages is not typical 



-See R. Kilkson and M. F. Maestre (1962). 



700 A 



HEAD 



MAIL 



figure 26-1. Diagrammatic representation of 

 the structures observed in intact and triggered 

 T-even phages of E. coli. 



