384 RICHARD SCHWKKT AND .lOlIX lUSIIOl' 



ent, otherwise the rate of enzyme synthesis would inci'ease with time. 

 Two exphinations arc considei'cd: (/I stahh' teniphites are iorme(l, hut 

 the gene heeouies inoperative after a few minutes of temphite formation; 

 (2) unstable temphites are formed whieh break down and are reformed 

 at some constant rate. Using P'--hibele(l donor bacteria carrying the 

 z* gene, it was found that breakdown of the genome due to P'^" decay, as 

 originally noted by McFall et al. (19581, led to decreases in ^-galacto- 

 sidase synthesis. These experiments appear to support the second possi- 

 bility. The concept of a rapidly formed intei-mediate which is the carrier 

 of information from DNA in the case of induced enzyme formation is 

 supported by many types of evidence. However, that this information is 

 messenger RNA, or RNA at all, has not been shown, in contrast to the 

 case of phage infection. Gros et nl. (1961b) have reported small increases 

 in RNA labeling on induction, which may represent such synthesis. The 

 evidence that an intact gene is required for /?-galactosidase synthesis 

 based on P^- decay cannot be interpreted unequivocally, particularly 

 since McFall (1961) has suggested that this effect results from repression 

 of enzyme synthesis by catabolites which accumulate in slowly growing 

 non-viable cells. Thus, the argument for an unstable template which 

 needs to be re-formed by continuous gene action, via messenger RNA 

 synthesis, rests largely on genetic studies and the kinetics of enzyme 

 formation after introduction of DNA into the cell. Another powerful 

 argument for the messenger RNA hypothesis in induced enzyme syn- 

 thesis is that the mechanism of induction is explained on the same basis, 

 e.g., the template is unstable and needs RNA synthesis continually; in 

 the absence of inducer, this synthesis is repressed. Thus, the inducer acts 

 at the genetic level to derepress messenger RNA synthesis (see Jacob 

 and Monod, 1961, for details of this hypothesis). 



In addition to the evidence given above, it has long been known that 

 RNA synthesis is needed for protein synthesis in bacteria. In part, these 

 studies have involved measurement of protein synthesis using purine- 

 and pyrimidine-requiring mutants. If RNA synthesis was limited by the 

 absence of the required base, protein synthesis was depressed (see Berg, 

 1961, for discussion). The interpretation of this RNA-protein relation- 

 ship is not clear. Suggestions have been made that common intermediates 

 for RNA and protein synthesis (amino acid-nucleotides) are present. 

 However, more indirect relationships due to jioor cell growth (McFall, 

 1961) may explain this apjiarent coupling of RNA and protein syn- 

 thesis (see Spiegelman et al., 1955, for an interesting summary of this 

 problem). More recently, the use of base analogs has permitted addi- 

 tional insight into the RNA-protein relationshij). In particular, 5- 

 fluorouracil (see below), azaguanine frhantr(nme. 1959). and thiouracil 



