ROLE OF NUCLEIC ACIDS 59 



Lactobacillus acidophilus suppresses DNA synthesis, but the bacteria con- 

 tinue to make RNA and protein. No division occurs, and the bacteria grow 

 into long filaments. Under conditions which prevent any measurable 

 synthesis of DNA, the amount of protein can increase by a factor ten 

 (Jeener and Jeener, 1952). Similar observations were made later by Cohen 

 and Earner (1955) with thymidine requiring strains of E. coli. In these 

 experiments the synthesis of enzymes was studied, and the results clearly 

 showed that perfect enzymatically active proteins can be made in the 

 absence of DNA synthesis. Very small doses of ultraviolet light (257 mja) 

 inhibit DNA synthesis without preventing protein formation (Kelner, 

 1953; Kanazir and Errera, 1954). High doses of X-rays can abolish DNA 

 synthesis completely without inhibiting the formation of enzymes, in 

 bacteria and yeast (Baron et ah, 1953; Chantrenne and Devreux, 1959). 

 Mustard gas also inhibits DNA synthesis in E. coli, without impairing 

 j8-galactosidase formation (Pardee, 1954), and the slight inhibition of 

 protein synthesis by N-mustard, which is observed occasionally, is due to 

 general toxicity (Clark et al., 1957). 



All these data constitute good evidence that the synthesis of DNA is not 

 necessary for protein synthesis in bacteria. This conclusion is an interesting 

 one, but more important still would be to establish whether the structural 

 integrity of the DNA gene is necessary for continued protein production, 

 since information relative to protein primary structure is contained in 

 DNA. 



When micro-organisms are irradiated with heavy doses of X-rays, their 

 DNA may be damaged. DNA extracted from yeast which had received 

 200,000 r was degraded in such a way that it could not be precipitated any 

 more by acid (Chantrenne, 1958; Chantrenne and Devreux, 1959). This 

 depolymerization of DNA may not be a direct effect of ionizing radiations; 

 it may be due to the action of some nucleases released in the cell by mem- 

 brane breakdown (F. Hutchinson, personal communication). Whatever the 

 mode of action of the X-rays may be, yeast cells containing largely degraded 

 DNA were able to produce proteins at a normal rate ; actually the synthesis 

 was somewhat stimulated. 



The first studies on the effect of DNA removal or DNA destruction on 

 protein synthesis in bacterial systems are due to Gale. Staphylococcus 

 aureus cells disrupted by supersonic vibrations are able to make enzymes 

 (Gale and Folkes, 1955). It is possible to remove nucleic acids (both 

 RNA and DNA) from such preparation by M NaCl extraction at 37° 

 and the degree of nucleic acids depletion can be controlled to a certain 

 extent. It was found that removal of the nucleic acids progressively reduces 

 enzyme production. The synthesis of various enzymes can be restored by 

 adding RNA if the depletion has been moderate, but DNA becomes an 

 additional requirement when nucleic acid depletion has been severe. This 



