INTEACELLULAR MULTIPLICATION OF BACTERIAL VIRUSES 251 



(Maal0e and Sjinoiids, 1953; Hershey et al., 1955; Watanabe, 1957). At least a 

 fraction of these incomplete sulfurylated phage antigens must be a precursor 

 of the mature phage progeny; not only are the antigens found to make their 

 intracellular appearance prior to that of any infective particles, but it can 

 also be showTi by means of experiments in which the S^^ label is either added 

 to or withdrawn from the infected culture at various times of the latent period 

 that some sulfur first present in incomplete antigens is later incorporated into 

 intact viruses (Maaloe and S}Tnonds, 1953; Hershey et al., 1955). 



2. Serum-Blocking Power 



One may search for the presence of "incomplete" phage antigens, not only 

 by virtue of their precipitability by antiphage serum, but also by virtue of 

 their ability to mte//ere with the antiserum neutralization of a test population of 

 the same phage strain. For any substance which has the same antigenic struc- 

 ture as the neutrahzing antigen of the phage can combine with and absorb out 

 the neutralizmg antibodies of an antiphage serum, thus diminishing the neut- 

 rahzing potency of the serum. The amount of such serum-blocking j)ower, or 

 SBP, can be estimated by comparmg the reduction of the neutralizing titer 

 produced by prior treatment of the serum with an unknown preparation with 

 that produced by treatment with a known number of intact phage particles. 

 One phage equivalent of SBP thus corresponds to the reduction in neutrahz- 

 ing activity produced by a single phage particle. Burnet (1933) discovered 

 that after removing the infective particles from T-even lysates^ by ultra- 

 filtration, there is still SBP material present in the filtrate. These observations 

 were confirmed by DeMars (1955) who found that from 10 to 25 % of the 

 total SBP of a T-even lysate is m an "incomplete" form, i.e., does not form 

 part of intact phages, being ultrafiltrable and nonsedimentable in centrifugal 

 fields which would bring down phage particles. DeMars also examined the 

 intracellular appearance of SBP in bacteria infected wdth T-even phages 

 under one-step growth conditions and lysed artificially at various times after 

 the onset of phage development. This experiment showed that the first, 

 nonsedimentable SBP appears between 9 and 10 minutes after infection, or 

 2-3 minutes prior to the first infective progeny. At later times, the SBP 

 activity was found to continue to increase along with the rise in progeny 

 titer, although the total amomit of SBP is always in excess of that attributable 

 to intact phages. Since the neutralizing antigens reside in the tail of the T-even 

 particle (Lamii and Lanni, 1953), it would seem that the "incomplete" SBP 

 structures detected in these experiments represent parts of the phage tail 

 protein. As such they are probably included in the antiserum precipitabie 

 surplus antigens described in the preceding section, 



^ Burnet's experiments actually concerned phage CI 6, which is closely related to the 

 T-even group (Adams, 1952), 



