60 THE BIOSYNTHESIS OF PROTEINS 



would indicate that DNA is required for the synthesis of certain enzymes. 

 The results, however, would be very interesting if the system required a 

 quite specific type of DNA. In some experiments it looked as if DNA from 

 the same bacterial species only was able to reactivate protein synthesis. 

 However, substances were later found among the split products of yeast 

 RNA which are able to reactivate the system just as well as nucleic acids. 

 The substances involved have already been purified to a considerable 

 extent by Gale but they are not as yet identified. They are low-molecular- 

 weight substances of very high activity. The meaning of these very 

 intriguing results is not clear at present (Gale, 1956, 1957, 1958); the 

 situation in this system reminds one to some extent of the case of isolated 

 thymus nuclei mentioned before; an agent which destroys DNA sup- 

 presses protein synthesis, but this process can be restored by other 

 substances beside DNA. Other experiments which are in some way com- 

 parable to Gale's were made by Spiegelman on protoplasts of B. mega- 

 teriiim (Spiegelman, 1957; Landman and Spiegelman, 1955). Lysozyme 

 catalyses the hydrolysis of cell walls of various bacteria. When living B. 

 megaterium or B. siibtilis are treated by lysozyme, their cell wall dissolves 

 more or less completely; if the osmotic pressure of the medium is suffi- 

 ciently high, the bacterial content is not dispersed in the medium and the 

 bacterial body changes into spherical droplets, the protoplasts (Weibull, 

 1953), which preserve most of the biochemical activities of intact bacteria, 

 including the synthesis of proteins (Lester, 1953; Beljanski, 1954; Bridoux 

 and Hanotier, 1954; McQuillen, 1955), including active enzymes (Wiame 

 et al., 1955), and bacteriophage production (Brenner and Stent, 1955; 

 Salton and MacQuillen, 1955). Treatment of the protoplasts by deoxy- 

 ribonuclease under suitable conditions (Spiegelman, 1957) results in the 

 removal of up to 99 per cent of the DNA; this has no adverse effect on 

 enzyme synthesis, which is often stimulated. Since deoxyribonuclease 

 splits the DNA molecule into very small pieces, it is obvious that the 

 physical integrity of DNA is not required for enzyme production in these 

 protoplasts. Spiegelman went a step further in the dissociation of the 

 system in using protoplasts which had been broken by osmotic shock. 

 From the disintegrated protoplasts, sedimentable systems were obtained 

 which retained the ability to make enzymes. Like Gale's disrupted Staphy- 

 lococci, these preparations consisted of membranous bodies containing only 

 a small fraction of the original DNA of the protoplasts. Treatment by 

 deoxyribonuclease removed most of the residual DNA and did not impair 

 enzyme synthesis. It must be mentioned, however, that much DNA was 

 reformed in these disrupted preparation at the same time as enzymes were 

 produced. 



It would seem that DNA is not a rate limiting factor of protein synthesis 

 in disrupted bacterial preparation since DNA can be damaged and elimin- 



