VIII. PROTEIN SYNTHESIS AND GENE ACTION 385 



have been used. Naono and Gros (1960a) and Biissard et al. (1960) 

 have studied the synthesis of ;8-galactosidase and alkaline phospha- 

 tase in E. coli grown in the presence of the analog, 5-fluorouracil. 

 Very soon after the introduction of the analog, the cells began to syn- 

 thesize altered enzymes which could be characterized by their immuno- 

 logical and enzymatic properties (yS-galactosidase) or by thermolability 

 (alkaline phosphatase). The change from the synthesis of normal to the 

 synthesis of altered enzyme was abrupt and apparently complete. These 

 results were explained by the hypothesis of Chantrenne (1959) that 

 certain nucleic acid analogs may be incorporated into the RNA of the 

 growing cell and exert their influence on protein synthesis by altering 

 the specificity of the RNA. Naono and Gros (1960a; see also Gros, 1960) 

 have observed that, within 1 minute of the addition of the analog, the 

 rates of incorporation of tyrosine and proline into protein dropped while 

 the rate of incorporation of arginine increased. An explanation of this 

 was found in changes in the saturation levels of these amino acids in 

 the amino acyl-RNA pool. Gros and Naono (1961), on the other hand, 

 have found additional changes in another RNA fraction. After incubation 

 of bacteria in the presence of C^^-5-fluorouracil, the radioactivity was 

 found exclusively in a fraction with the sedimentation coefficient 12-14S. 

 After phenol extraction of the RNA, the radioactivity sedimented at 

 8-lOS. The suddenness and completeness of the effect provide good 

 evidence for a rapid synthesis of the RNA fraction involved, and the 

 sedimentation data suggest identity with messenger RNA. Thus, the 

 changes in enzyme synthesis are explained by the synthesis of an altered 

 messenger RNA containing the analog. The problem is somewhat com- 

 plicated by the finding of incorporation into other types of RNA at 

 later time periods (see Chantrenne and Devreux, 1960, for example) 

 with azaguanine, and the variation in reversal of inhibition when guanine 

 is added after azaguanine incubation for various times. 



The final topic for discussion in this section is the "instability" of 

 the messenger RNA. Two types of instability, which may be related, 

 can be distinguished. The first concerns the question of whether mes- 

 senger RNA breaks down to the nucleotide level before assimilation into 

 ribosomal RNA, or is used at the polynucleotide level. This question is 

 discussed in detail by Roberts (Chapter VII) and by Volkin (Chapter 

 VI). A few observations will be made here. The RNA fraction under 

 consideration in a particular experiment may vaiy. When, for example, 

 the criterion for messenger RNA, is that of a rapidly labeled RNA 

 fraction, this may contain various types of RNA. Part of this may be 

 RNA destined for ribosome synthesis; another part may be messenger 

 (defined as carrying infomiation for protein sequence determination). 



