VIII. PROTEIN SYNTHESIS AND GENE ACTION 381 



now considered to be via formation of a "messenger" RNA. Since the 

 problem of DNA and messenger RNA synthesis is reviewed elsewhere 

 (see Chapters VI and VII), the emphasis here will be on the function of 

 DNA and RNA in i:)rotein synthesis, particularly in cell-free systems. 

 The question of whether a linear sequence of the bases in DNA cor- 

 responds to a linear sequence of amino acids is omitted also. The role 

 of RNA in protein synthesis in cell-free systems will be emphasized, 

 since studies in intact bacteria are reviewed by Volkin (Chapter VI). 

 Finally, results with bacteria are treated separately from those using 

 higher organisms because of the very different picture of DNA function 

 in these two cases. 



A. INFORMATION TRANSFER IN BACTERIAL PROTEIN SYNTHESIS 



The evidence that DNA carries the information for protein synthe- 

 sis in most systems (RNA viruses may be an exception) is clear. The 

 question to be considered here is the mechanism involved. One possibility 

 might be that DNA is itself the template for protein synthesis. This 

 does not appear to be the case, although in intact bacteria the evidence 

 is not particularly convincing. Rather, the ability of enucleated algae 

 and amebae to continue protein synthesis has provided the major evi- 

 dence for this view (see next section). The best evidence that DNA is 

 not the template for protein synthesis in bacteria is the finding that 

 DNA acts to make a particular type of RNA, which does appear to 

 participate in the formation of the template for protein synthesis, as 

 discussed below. These studies involve cell-free systems and it must be 

 noted that the evidence using intact bacteria does not distinguish clearly 

 between the direct action of DNA as a template, or the accepted 

 hypothesis that DNA produces an unstable intermediate, the messenger 

 RNA. 



The evidence which has led to the present concept of an unstable 

 "messenger" RNA as the intermediate between DNA and protein comes 

 largely from studies of induced enzyme formation (Jacob and Monod, 

 1961) and phage infection (see Chapter VI). The existence of RNA 

 which is rapidly labeled in intact cells, has the composition of DNA 

 and can form what seem to be specific hybrids with DNA has been 

 demonstrated. These studies have relied largely on labeling experiments 

 and the isolation and purification of messenger RNA has not been 

 achieved, although the report of Bautz and Hall (1962) suggests that 

 this may be accomplished in the near future. However, crude messenger 

 RNA isolated from the intact cell is heterogeneous wdth regard to 

 molecular weight (see Chapter VI for detailed discussion). This hetero- 

 geneity may reflect a structural heterogeneity related to the particular 



