386 UICII.MU) SCinVKKT AND JOHN HlSIIOl' 



Secontl, tlie type of exporiiiient may. in fact, (Ictci'iniiic the fate of the 

 RNA. For example, in pliage-infected cells, wlicic miclccjtides are used 

 for synthesis of deoxynucleotides, and ribosomc synthesis is inhibited, 

 the breakdown of messenger RNA may be emphasized over other path- 

 ways. These points are mentioned because of the a])i:)arent disagreement 

 in the literature on this question. In any event, it seems that under some 

 conditions there is extensive breakdown of the labeled RNA fraction. 

 In some cases, the specific RNA fraction is loosely bound to ribosomes, 

 but another part is more firmly incorporated in ribosomes which have 

 been described as "active" 70S (and also probably lOOS) ribosomes (see 

 below). This RNA is not removed by washing in 10 ' .1/ magnesium 

 chloride (Gros et ciL, 19611)) and may be the functionally important 

 form of messenger RNA. Finally, a third form may involve synthetic 

 processes which result in the labeled RNA being changed to ribosomal 

 RNA as regards base composition. In this situation it may be func- 

 tionally inactive. 



The important ciuestion with regard to functional stability of mes- 

 senger RNA is related to the mechanism by which it controls protein 

 synthesis. It has been suggested (Gros et al., 1961b) that there is a 

 stoichiometric breakdown of messenger RNA during protein synthesis. 

 This would result if RNA breakdown were directly coupled to jieptide 

 bond synthesis. There is little evidence to sui)port such a mechanism. 

 Particularly, in reticulocytes (Bishop et nl., 1961, and see below) the 

 stability of messenger RNA rules out such a mechanism. Even in bac- 

 teria, there is evidence that a considerable variation in the functional 

 stability of messenger RNA exists. The extreme instability in the case 

 of phage infection and induced enzyme synthesis has been noted. How- 

 ever, even in phage infection, at a later stage the synthesis of messenger 

 RNA is low, while phage synthesis continues (Astrachan and Volkin, 

 1958). 



2. Function of DNA and RNA in Cell-Free Systems 



The cell-free system for incorporation of amino acids developed by 

 Lamborg and Zamecnik (1960) was the first ribosomal system from 

 bacteria. This cell-free system was similar in many respects to cell-free 

 systems from animal tissues. A remarkable property of this system, not 

 found in animal systems (see below), was inhibition by DNase. Tissieres 

 et al. (1960) obtained uji to 70% inhibition of amino acid incorporation 

 with DNase. RNase contamination was excluded and polyelectrolytes 

 did not reverse the inhibition, suggesting that removal of DNA was 

 the cause of inhibition. One of the synthetic compounds used in an 

 attempt to restore activity was poly-A, which was ineffective. Inhibition 



