CHEMISTRY AND VIRAL GROWTH 

 VIRAL PRECURSOR PROTEIN 



Cohen (7) found that there was no interruption of protein 

 synthesis at the time of infection, and that protein continued to 

 accumulate until viral growth ceased. His findings have been 

 confirmed by measuring the assimilation of S^^ into acid-in- 

 soluble materials after infection (29). These measurements 

 show that bacteria synthesize protein at nearly identical rates 

 before, during, and for some time after infection. When does 

 the synthesis of viral protein start? 



This question has been attacked in two ways. Luria (44) 

 and his collaborators studied the formation of viral antigens in 

 infected cells. Their results led to the conclusion that viral 

 antigens are formed relatively late during the process of viral 

 growth, as though they were products of the maturation cycle. 

 If this is so, the protein synthesized immediately after infection is 

 not virus specific. 



Hershey et al. (29) analyzed viral precursor protein by 

 tracer methods. They fed S^^ to infected bacteria for intervals 

 of 5 minutes, and measured the amount of S^^ subsequently 

 incorporated into virus and total intrabacterial protein, re- 

 spectively. The amount incorporated into protein of all kinds 

 was virtually independent of the period of assimilation tested, 

 but the proportion of this incorporated into virus varied greatly. 

 For the period to 5 minutes after infection, only 13 per cent 

 went into virus; for 5 to 10 minutes, 25 per cent; for later times, 

 50 to 60 per cent. This means that infected bacteria synthesize 

 two classes of protein, one viral precursor and one not, and that 

 the maximal rate of precursor protein synthesis is not reached 

 until shortly before the beginning of maturation (15 minutes 

 after infection under the conditions of these experiments). 

 Kinetic analysis of sulfur assimilation during the period in which 

 virus was accumulating at a linear rate showed that the interval 

 between assimilation of sulfur atoms and their incorporation into 

 unspecified protein averaged about 2 minutes. The interval 

 between assimilation and incorporation into mature virus aver- 

 aged about 10 minutes. Hence labeled viral precursor protein 



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