38. BIOSYNTHESIS OF PROTEINS IN BACTERIAL CELLS 443 



tionship between the RNA content of a cell and the rate at which it syn- 

 thesizes protein. The second is based on metabolic studies of bacteria 

 whose RNA has been partially or completely destroyed by ribonuclease 

 treatment. 



It is known from Brachet's early observations 181 that the cells of an or- 

 ganism which are richest in RNA are the most rapid in growth and division. 

 While this observation had the great merit of drawing attention to the 

 possible role of the RNA in protein synthesis, it did not define, however, 

 the precise relationship between RNA and protein. More precise and 

 quantitative information might be obtained by trying to correlate protein 

 synthesis and RNA content in a given cell, rather than amongst cells of 

 different origin. This type of study has been made with animal cells 181 and 

 on a more extensive scale with bacteria by relating the growth rate to the 

 cellular RNA content. 



Earlier experiments involved an examination of the variation in the RNA 

 content in a bacterial culture at different growth phases. It became clear 182 

 that bacteria contain less RNA during the stationary phase than during 

 the exponential phase of growth. However, the interpretation of the rela- 

 tionship between growth rate and cellular RNA content during the transi- 

 tion from the lag period to the exponential phase is complicated by a series 

 of factors such as size variations in individual bacteria. 



A more satisfactory approach is to consider only bacteria in the steady 

 state (exponential phase), but exhibiting different growth rates according 

 to the nature of the carbon source or to the kind of supplements added to 

 the medium. This has been done by several authors. 139, 183, 184 The kind of 

 relationship obtained is illustrated in Fig. 5. 



For rapidly growing cells (growth constant K between 0.6 and 1.1) the 

 growth rate is proportional to the RNA content (expressed by the ratio 

 RNA/protein). For lower values of K (between 0.2 and 0.6) the relation- 

 ship is no longer linear and the RNA content varies very little while the 

 growth rate varies appreciably. 



Herbert 1 ' 85 has recently reported very similar results in studying the effect 

 of growth rate on nucleic acid and protein content, and mean cell mass of 

 A. aerogenes grown in a chemostat on a mineral medium with various limit- 

 ing concentrations of glycerol. 



The results are compatible with the assumption 139 that the protein-form- 

 ing system is composed of a ribonucleoprotein (or at least contains RNA 



181 J. Brachet, "The Nucleic Acids" (E. Chargaff and J. N. Davidson, eds.), Vol. II, 

 p. 475 Academic Press, New York, 1955. 



182 B. Malmgren and C. (1. Heden, Acta Pathol. Microbiol. Scand. 24, 437 (1948). 



183 P. C. Caldwell, E. L. Mackor, and C. N. Hinshehvood, J. Chem.Soc. p. 3151 (1950). 



184 R. Jeener, Biochim. et Biophys. Acta 8, 125 (1952). 



185 D. Herbert, 7th Intern. Congr. Microbiol., Stockholm p. 381 (1959). 



