Chapter *42 



VIRUSES: RECOMBINATION 

 IN BACTERIOPHAGE (I) 



1 



"t has been already stated that the 

 genetic material of bacterial chro- 

 .mosomes is DNA. It has been 

 inferred (p. 366) that the episome F is also 

 composed of DNA, and proof of this has been 

 obtained. The possibility may not be ex- 

 cluded at the present time, however, that some 

 episome other than F may have RNA for its 

 unique genetic material, for this substance, 

 too, could provide nucleotide sequences suit- 

 able for pairing with chromosomal segments 

 of DNA. It is even possible to imagine that 

 RNA might become integrated into a DNA 

 chromosome.^ (RNA episomal particles that 

 deintegrate from the chromosome and retain 

 a DNA segment as chromosomal memory 

 could subsequently readily reintegrate.) Since 

 we concluded near the close of the last 

 Chapter that most, if not all, temperate 

 phages are episomes, it would seem desirable 

 at this point to discuss the morphology, 

 chemical composition, and some details of 

 the behavior of bacteriophages. 



Different phages have different morpholo- 

 gies. Consider the structure of phages of the 

 T-even group (T2, T4, T6), since these have 

 been studied in some detail. ^ Such phages 

 are tadpole-shaped, 0.1-0.2/i long, or about 

 a tenth the bacterial diameter (see Figure 

 42-1). The head has the form of a bipyrami- 

 dal hexagonal prism, while the tail is cylindri- 



^ See reference to J. S. Krakow, H. O. Kammen, and 



E. S. Canellakis at end of Chapter. 



2 See reference to S. Brenner et at. (1959) at end of 



Chapter, 



382 



cal and is the structure used for attaching the 

 phage to the host cell. The membrane sur- 

 rounding the head is composed of a large 

 number of repeated subunits each having a 

 molecular weight of about 80,000. The tail 

 consists of an outer sheath which is composed 

 of spirally arranged subunits which form a 

 hollow cylinder. The sheath can contract in 

 length as a consequence of which its diameter 

 is increased but its volume is approximately 

 unchanged. The sheath is composed of about 

 200 subunits, each of approximately 50,000 

 molecular weight. Beneath the sheath is the 

 core of the tail. The core is a hollow cylinder 

 with a central hole about 25 A in diameter. 

 At the distal end of the core is a hexagonal 

 plate to which six tail fibers are attached. 

 Each tail fiber has a kink in the middle and 

 seems to contain subunits with a molecular 

 weight not less than 100,000. The head 

 membrane, sheath and tail fibers are com- 

 posed of proteins, each of whose digestion 

 with trypsin gives a unique fingerprint. 

 Therefore, each of these proteins is different. 

 The core also is composed of protein. A 

 serologically distinct protein, comprising 

 4-6% of the total protein, is found in the 

 interior of the phage particle; polyamines, 

 putrescme, spermadine, lysozyme, and a 

 minor polypeptide are also reported in the 

 phage interior. 



In addition to the components already 

 mentioned, the phage interior contains DNA 

 whose volume is approximately equivalent to 

 that of the total protein. This DNA is com- 

 posed of a double helix about 200,000 nucleo- 

 tides long. Since such a polynucleotide 

 would be about 68/x long, it is clear that the 

 DNA inside the phage must be highly coiled 

 upon itself. No RNA is reported to be 

 contained in these phages. However, not all 

 phages contain DNA. At least one phage is 

 known to contain RNA and no DNA. More- 

 over, the physical and chemical complexity 

 of the T-even phages is not typical of all 

 viruses. Certain plant viruses, like tobacco 



