:M } U. H. ROHKRTS. R. J. KHITTKN, AND U. J. MCCARTHY 



imist !)(,' liijj;lily active coinpaivd to the average DNA. The entire lac 

 region only comprises a tiny part of the chromosome, yet its product 

 must account for at least 3.6% of the gene-directed material. 



This example shows the present difficulties in correlating the facts 

 of ribosome synthesis with their probable role as enzyme-forming units. 

 Only relative enzyme-forming capacities are measured and several wild 

 assumptions (which can only be justified as the best guess at present) 

 are needed to estimate the absolute number of EFIJ per cell. 



C. MESSENGER THEORY 



At present the most precisely foi'inulated theory of the role of ribo- 

 somes in information transfer is the messenger hypothesis (Jacob and 

 Monod, 1961). The kinetics of enzyme induction and deinduction in- 

 dicate that enzyme-forming units are unstal)l(> and that template mate- 

 rial must be continuously supplied by the gene. However, the ribo- 

 somes which appear to be the sites of protein synthesis are stable. Ac- 

 cordingly, Jacob and Monod postulate that an unstable messenger RNA 

 is produced by the gene and associates temporarily with ribosomes to 

 act as template for protein synthesis. 



They predict that cells should therefore contain an RNA fraction 

 having the following properties: (a) average molecular weight 5 X lO"' 

 or more, (b) base composition reflecting the base composition of DNA, 

 (c) capable of associating with ribosomes, and (d) having a high rate 

 of turnover. 



As described in Sections V and VH, material having these character- 

 istics has been found both in experiments designed to identify "mes- 

 senger" RNA (Brenner et oL, 1961 ; Gros et al, 1961) and in experiment^ 

 designed to observe ribosome synthesis (McCarthy et ai, 1962). De- 

 pending upon the objective of the experiment the same material is desig- 

 nated messenger RNA or the eosome fraction. The quantity of this 

 material is between 1 and 3% of the ribosomal RNA. If its molecular 

 weight is 5 X lO-' (V, the molecular weight of the RNA of the 70S!) it 

 would be sufficient to provide 3-9% of the 70S ribosomes with tem- 

 plates; only a small fraction of the ribosomes could be active in protein 

 synthesis at any one time. There remains some question whether the 

 "turnover" is due to degradation to low molecular weight material or 

 to incorporation into ribosomes. 



The experimental evidence considered in formulating the messenger 

 theory was heavily weighted in favor of the rapid transients observed 

 during enzyme induction or phage infection. Quite a different predic- 

 tion of the properties of temjilate material could be drawn. Since the 

 amino acid composition of a series of organisms varies much less than 



