VT. RXA AND CODING PROBLEMS 275 



C. MESSENGER RNA-HOMOLOGGUS DNA HYBRIDS 



Perhaps the most convincing evidence for the existence of a phagc- 

 specific messenger RNA has been accomphshed by Hall and Spiegelman 

 (1961). These workers demonstrated that when single-stranded T2 DNA 

 was slowly cooled in the presence of partially purified T2-specific RNA, 

 an RNA-DNA hybrid was formed that could be separated by equilibrium 

 density centrifugation. Since such hybrid formation was found to be 

 unique to the homologous pair, the conclusion seems valid that a high 

 degree of homology exists between T2 DNA and T2-specific RNA in 

 terms of their complementaiy nucleotide sequential patterns. A later 

 study by Spiegelman et al. (1961) demonstrated that such DNA-RNA 

 complexes could be detected as naturally occurring hybrids in T2- 

 infected E. coll. Hayashi and Spiegelman (1961) have extended these 

 observations to bacterial systems that were subjected to a shift from an 

 enriched medium to a basal medium. These systems simulate conditions 

 of phage infection whereby ribosomal RNA synthesis is depressed while 

 protein synthesis and messenger RNA synthesis proceed preferentially. 

 The RNA synthesized and labeled under these "step-down" cultural 

 conditions possessed the ability to form RNA-DNA hybrids when 

 slow-cooled with only their homologous DNxA-'s. 



D. RNA TURNOVER 



The question of turnover or metabolic instability of messenger RNA, 

 although referred to later with respect to its functional role, warrants 

 further comment here. The findings of Yolkin and Astrachan (Volkin 

 and Astrachan, 1957; Astrachan and Volkin. 1959; Volkin. 1962) with 

 T2-infected E. coli demonstrate clearly that the T2-specific RNA is 

 labile and converted to acid-soluble products. Furthermore, these break- 

 down products do not. to any significant extent, reach the level of 

 nucleosides and inorganic phosphate (Volkin. 1962). The phosphorus 

 originating in messenger RNA can eventually terminate in phage DNA 

 but not by either an obligatory pathway or a pathway utilizing large 

 (acid-insoluble) polynucleotide fragments (Volkin. 1962). The in vitro 

 data of Cohen et al. (1961) would indicate that the phage-specific RNA 

 is preferentially enzymatically hydrolyzed to ribonucleoside-5'-P04's, 

 which are readily reutilized for phage DNA synthesis. 



In growing cultures of bacteria, a turnover of messenger RNA can be 

 demonstrated with respect to (1) its transient existence evidenced by 

 isotopic labeling, and (3) its geographical translocation from one type 

 of ribosomal particle to another. It should be noted, however, that there 



