382 RICHARD SCHWEET AND JOHN BISHOP 



protein for wliich the RXA contains the sequence information. The 

 separation of the postulated single protein-specific messenger RNA has 

 not been achieved. Further, RNA with the i)roperties described above 

 may be heterogeneous with regard to function and need not consist 

 solely of RNA carrying sequence information for protein synthesis (see 

 Hayashi and Spicgelman. 1961, for a discussion of tiiis point). The 

 synthesis of RNA with the base composition of a primer DNA has been 

 demonstrated in cell-free systems (Weiss, 1960; Weiss and Nakamoto, 

 1961; Hurwitz et ai. 1960; Stevens, 1960). The use of these "RNA 

 polymerase" enzymes in cell-free systems has provided the most powerful 

 evidence for the messenger RNA hypothesis. 



1. Functioning of Messenger RXA in Intact Cells 



Among the first clues which suggested that RNA could carry informa- 

 tion for protein synthesis was the finding of infective TMV-RNA (Gierer 

 and Schramm, 1956; Fraenkel-Conrat, 1956). At the time, the idea that 

 this infective RNA played a direct role in the synthesis of TMV coat 

 protein and other proteins was not seriously considered. Recent results in 

 cell-free systems (see below) and the effect of mutagenic agents on 

 TMV-RNA (see Chapter X) suggest that TMV-RNA is or contains 

 messenger RNA. However, the molecular weight of infective T]\1V is 

 higher than the reported values for the sedimentation constants of 

 messenger RNA. which appear to range from 8 to 16S.^ The size of the 

 minimum unit of T]MV which will act as messenger in cell-free systems 

 is not known. 



The other systems which have led to the messenger RNA concept also 

 involve "new" protein i^ynthesis. The very careful studies of Volkin and 

 Astrachan (see Chapter VI) showed that following phage infection of 

 E. coli a small fraction of the RNA was labeled when P^- was added. 

 This RNA had the properties of messenger RNA, particularly with 

 respect to the base composition which resembled the DNA of the 

 infecting phage. Recently, Volkin has shown that the synthesis of this 

 RNA is essential for phage protein synthesis and, in fact, that some 

 stoichiometry exists between the amount of RNA and jirotein synthesis. 

 This kind of evidence and similar studies which, for example, show a 

 requirement for uracil for phage-induced enzyme synthesis (Pizer and 

 Cohen, 1961) provide strong evidence that a particular type of RNA 

 must be synthesized prior to phage protein synthesis. However, such 

 studies do not define the exact function of this RNA. 



The function of this type of RNA, as a component of the template 



'More recent results indicate the sedimentation constants of' messenger RXA to 

 range from 8 to 308 (F. Gros, private communication). 



