APPENDIX 505 



infected with a cofactor-requiring variant will produce a plaque 

 on a tryptophan-free medium while an unadsorbed phage par- 

 ticle will not. The efficiency of plating of the infected bacte- 

 rium is enormously higher than that of the free phage particle. 

 The reason for this is not clear, but knowledge of the fact has 

 permitted study of the rate of formation and of decomposition of 

 the tryptophan-phage complex under various conditions (T. F. 

 Anderson, 1948a). This study has been facilitated by Ander- 

 son's ''dumping" technique, a description of which follows. 



Procedure. A sample of phage T4 is suspended in a chemically 

 defined medium containing 10 /xg./ml. of tryptophan and the 

 mixture incubated until all of the phage has been "activated," 

 i.e., has formed a complex with tryptophan. A 0.1 ml. sample of 

 the activated T4 phage is placed in a large test tube and 10 ml. of 

 a suspension of B in a tryptophan-free medium is dumped in 

 from a second test tube so that the tryptophan is diluted 1 : 100 at 

 the same moment that the bacteria are added. The proportion 

 of T4 phage which is adsorbed onto the bacteria is a function 

 both of the rate of adsorption of activated phage onto bacteria 

 and of the rate of deactivation of phage by dissociation of the 

 phage-tryptophan complex. An estimate of the rate of deactiva- 

 tion is made by dumping in a large volume of tryptophan-free 

 medium at zero time and, after allowing deactivation to proceed 

 for a measured time, dumping in the bacterial suspension and 

 permitting the residual activated phage to adsorb onto bacteria. 

 In all instances the number of infected bacteria is then deter- 

 mined by plating on tryptophan-free medium, on which the in- 

 fected bacteria produce plaques but the free phage particles do 

 not. Under most conditions the rate of deactivation is more 

 rapid than the rate of adsorption, so that only a fraction of the 

 activated phage-tryptophan complex is registered by this 

 method. 



3. Plaque Morphology Mutations 



A thoroughly studied plaque morphology mutation occurs in 

 the serologically related family of coliphages T2, T4, and T6. 



