294 NUCLEIC ACIDS AND GROWTH 3 



hybrids, DNA synthesis can go on in the absence of protein synthesis, RNA synthe- 

 sis and growth. Present evidence for micro-organisms indicates that the converse 

 situation is possible, i.e DNA synthesis can be inhibited without interference of 

 protein synthesis, RNA synthesis and growth; furthermore, inhibition of protein 

 and RNA synthesis without any effect on DNA synthesis, as in the lethal hybrids, 

 can be obtained when bacteria are treated with a chemical analogue of phenyl- 

 alanine (Pardee and Prestidge, 1955). 



Cytological observations by Jeener and Jeener (1952) have shown that, in 

 Therrnobacterium acidophilus, suppression of DNA in the culture medium stops 

 nuclear multiplication, but not growth: the result is the formation of filamentous 

 bacteria, containing only a small number of nuclei. When uracil is absent from 

 the medium, synthesis of RNA is stopped : the result, this time, is an inhibition of 

 growth. 



A more complete biochemical analysis of a comparable case has been made by 

 Cohen and Earner (1954), who studied the effects of thymine deficiency on a 

 thymine-requiring mutant o( Escherichia coli: the organism loses the power to form 

 colonies, and this type of sterilization is accompanied by a marked increase in 

 bacterial length and girth and doubling of the RNA content; but there is almost 

 no synthesis of DNA. 



Of special interest is the fact that these bacteria, which lack an appreciable 

 DNA synthesis, are nevertheless capable of induced enzyme synthesis (synthesis of 

 xylose isomerase in response to the presence of xylose in the medium). These 

 experiments clearly show that cytoplasmic and induced enzyme synthesis are 

 possible in the absence of appreciable DNA synthesis. 



Similar conclusions are reached when the effects of various physical and chemi- 

 cal agents on bacteria cultures are studied: for instance, Kelner (1953) and Kana- 

 zir and Errera (1954) have independently shown that low doses of U.V. light have 

 little effect on RNA synthesis and growth, although they completely stop DNA 

 synthesis. As in the case studied by Jeener and Jeener (1952), the result is the ap- 

 pearance of filamentous bacteria. 



It should however be pointed out that, according to Swenson (1950), U.V. light 

 inhibits the induced synthesis of galactozymase in yeast, the action spectrum being 

 similar to a nucleic acid absorption spectrum. This finding, together with that of 

 Kelner (1953) and Kanazir and Errera (1954) on the immediate inhibition of 

 DNA synthesis by U.V. light, might point toward a direct nuclear control of 

 induced galactozymase synthesis. But more recent investigations of Halvorson 

 and Jackson (1954) indicate that the U.V. doses used by Swenson were too high 

 to obtain a dissociation between DNA synthesis and growth; they further found 

 that marked effects on protein synthesis are obtained when slight alterations occur 

 in the RNA molecules. The conclusion of Halvorson and Jackson (1954) is, again, 

 that suppression of DNA synthesis does not inhibit protein synthesis; the latter is, 

 on the other hand, strongly dependent on the integrity of the RNA molecules. 



It is a well established fact that DNA and DNA synthesis are much more sensi- 

 tive than RNA to such agents as X-rays and mustards: it is therefore not sur- 

 prising that Baron et al. (1953) found X-rays to be ineffective in inhibiting induced 

 enzyme synthesis and that Sher and Mallette (1954), Pardee (1954) and Gros et 



