REQUIREMENTS FOR PHAGE PRODUCTION 261 



precursor nor the mature phage exchanges phosphorus with 

 the phosphate of the medium. The phosphorus of mature phage 

 does not exchange with the phosphorus of the precursor DNA 

 indicating that maturation is an irreversible process. Matura- 

 tion is a remarkably efficient process ; about 90 per cent of the 

 phosphorus which enters the pool early is later incorporated into 

 phage. Phage DNA is synthesized at the rate of 7 to 8 phage 

 particles per minute per bacterium; this is faster than bac- 

 terial DNA but slower than bacterial RNA is produced in un- 

 infected bacteria. The transport of phosphorus from medium 

 to precursor DNA takes an average of 8 or 9 minutes, and Irom 

 precursor to mature phage an additional 7 or 8 minutes. The 

 usable parental phosphorus enters the precursor DNA pool be- 

 fore 10 minutes. The parental phosphorus is incorporated into 

 mature phage progeny between 10 and 25 minutes after infec- 

 tion. The preassimilated phosphorus from the host cell enters 

 the precursor pool at the same rate as the bacterial DNA dis- 

 appears. It has essentially all entered the pool by 25 minutes 

 and has been incorporated into mature phage by 40 minutes after 

 infection. Hershey suggests that the rate of phage maturation is 

 determined by the pool size and hence by the rate of synthesis of 

 phage-precursor DNA. 



This demonstration of the properties of the pool of phage-pre- 

 cursor DNA is of great interest because of the resemblance of this 

 pool to the vegetative phage pool, or mating pool, discerned in 

 genetic experiments (Chapter XVIII). When a bacterium is in- 

 fected with two or more genetically distinguishable particles of 

 phage T2, the phage nucleic acids penetrate into the host cell 

 and are then called vegetative phage. The vegetative phage 

 particles multiply, and when the intracellular population be- 

 comes high enough for unlike pairs to collide, mating begins. 

 After this time vegetative phage is withdrawn from this mating 

 pool at a linear rate to form mature phage. Mature phage 

 particles play no further role in replication or mating. The 

 mating pool contains a constant number of 30 to 50 vegetative 

 phage particles. It seems reasonable to conclude (Hershey, 



