74 THE BIOSYNTHESIS OF PROTEINS 



necessary in each case. This situation may look depressing; on the other 

 hand, it makes it possible to find organisms and analogues which will be par- 

 ticularly suitable for the study of a given biochemical problem. Cases which 

 are especially interesting for the problem considered in the present chapter, 

 namely the importance of RNA integrity for protein synthesis, are those of 

 organisms which incorporate purine or pyrimidine analogues into their 

 nucleic acids. Thus 8-azaguanine is incorporated into RNA of various 

 organisms (Mitchel et al, 1950; Heinrich et al, 1952; Bennett et ah, 1953; 

 Mandel et al, 1954; Matthews, 1953, 1957; Lasnitski et al, 1954; Mat- 

 thews and Smith, 1955, 1956; Mandel and Markham, 1958). 2-Thiouracil 

 incorporation into RNA of tobacco mosiac virus (Jeener and Rosseels, 

 1953; Jeener, 1957; Matthews, 1956; Mandel et al, 1957) and of Bacillus 

 megaterium (R. Hamers, 1956; Amos et al, 1958) has been established. 

 5-Fluorouracil can also be incorporated into RNA in several organs of rat 

 and mouse (Heidelberger et al, 1957; Harbers et al, 1959), in Escherichia 

 coli (Horowitz et al, 1958; Horowitz and Chargaff, 1958; Gros, 1959; 

 Brockman et al, 1960) and in tobacco mosaic virus (Gordon and Staehelin, 

 1959). 



As 2-thiouracil is being incorporated into RNA by Bacillus megaterium, 

 the bacteria continue to grow and to make proteins, but growth becomes 

 linear (Hamers, 1956). In E. coli, a similar change in the shape of the 

 growth curve is observed, but the effect is not as rapidly established as in 

 B. megaterium. Protein synthesis is but slightly inhibited by thiouracil in 

 E. coli, but the production of certain enzymes is drastically reduced. The 

 differential rate of increase in j8-galactosidase activity, for instance, is 

 reduced by about 90 per cent. The residual increase is due to the produc- 

 tion of a protein material which is precipitated by an antiserum prepared 

 against regular j8-galactosidase. But titration by the immunological method 

 of Cohn and Torriani (1952) shows that this material has less enzyme 

 activity per unit of serologically precipitable protein than regular j3- 

 galactosidase. This indicates that a slightly abnormal enzyme is produced 

 in the presence of thiouracil (Hamers and Hamers-Casterman, 1959). 



Bacteriophage production by a lysogenic strain of Bacillus megaterium 

 can also be inhibited by 2-thiouracil. The major action of the analogue in 

 the present case is a drastic inhibition of phage DNA synthesis; the 

 formation of phage proteins is also decreased, but to a lesser extent, and the 

 protein produced in the presence of thiouracil retain normal immunological 

 properties, as well as the capacity of fixation upon bacteria (Jeener et al, 

 1959). 



5-Fluorouracil inhibits growth of £". coli by 50 per cent. Certain enzymes, 

 like catalase, succinic dehydrogenase or phosphatase continue to be pro- 

 duced whereas j3-galactosidase synthesis is almost completely stopped 

 (Horowitz et al, 1958; Horowitz and Chargaff", 1959; Gros, 1960). While 



