276 KLLIOT VOLKIX 



is uniform and ox-ci-wlicliniiiti; evidence that a significant measurable loss 

 of acid-insoliihle lor liitili molecular weight) RXA ne\-er occuis in 

 normally growing bacteria. Thus, in pulse-type experiments, after re- 

 moval of tlic radioactive substrate, instead of a loss of label, there is a 

 continued increase in radioactivity into total RNA for some time. 

 Concurrently, there is a shift in labeling pattern of the RNA nucleotides 

 in a manner tliat would indicate an over-all transition of DNA-like 

 RNA to ribosomal, or total, RNA (Volkin and Astrachan, 1957; Hayashi 

 and Spiegelman, 1961). These observations pose the question of the fate 

 of messenger RNA and its jirobable transformation to stable ribosomal 

 RNA. This aspect of the i)roblem is covered in more detail in Chapter 

 VII. Suffice it to note here that since most organisms and tissues contain 

 ribosomal RNA of high cytidylic and particularly high guanylic acid 

 contents, efficient conversion of messenger RNA to ribosomal RNA 

 could indicate that large amounts of guanylic and cytidylic acids have 

 been added. Kitazume et al. (1962) studied the kinetics of synthesis of 

 the dRNA [sic] in yeast subjected to differing metabolic conditions. 

 They conclude that dRNA is an obligatoiy precursor to i-ibosomal RNA 

 and that this transition occurs by a subtraction of nucleotides from 

 dRNA. They further propose that instability of dRNA is not necessarily 

 a result of its biologic action (see below) but merely results from con- 

 version to ribosomal RNA. 



E. MESSENGER RXA IN OTHER BIOLOGIC SYSTEMS 



Yeas and Vincent (1960), comparing the labeling of RNA mono- 

 nucleotides liberated as the 2'-, 3'-isomers (alkaline hydrolysis) and as 

 the 5'-nucleotides (phosphodiesterase hydrolysis), concluded that in 

 exponentially growing yeast an RNA with a composition like yeast 

 DNA exists. Astrachan and Fi.-^hcr (1961) and Volkin (1962) used 

 30-second pulses of P'--labeling to demonstrate the existence of a 

 DNA-like RNA in Proteus vulgaris, E. coli, anfl Psendomonas aerugi- 

 nosa — AT/GC ratios of these organisms vary from 1.47 to 0.92 to 0.50, 

 respectively. In such growing bacteria, if the isotope incorporation 

 period is extended to a few minutes, the labeling pattern appears to be 

 identical with total (or ribosomal) RNA, in accord with the observa- 

 tions that messenger RNA is present as a minor fraction of total RNA. 

 The occurrence of a messenger RNA in E. coli and P. aeruginosa has 

 been independently noted by Hayashi and Spiegelman (1961). who 

 likewise determined its presence in Bacillus megaterium, an organism 

 with DNA of high AT/GC ratio. The observations of messenger RNA in 

 growing E. coli by Gros et al. (1961) have been mentioned. 



Some evidence exists for the presence of an RNA with a composition 



