BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 491 



been reported by Levy et at. :^^ addition to growing Proteus of 2 X 10~^ M 

 cobaltous sulfate stops growth without arresting PNA synthesis. Miura 

 and his co-workers^^'^® have also reported comparable findings when bac- 

 teria are treated with the antibiotic usnic acid. The significance of these 

 observations, which have all been made in the presence of antibiotics which 

 interfere with normal growth of the microorganisms, will be discussed in a 

 later section. 



The very great importance of the culture conditions in such experiments 

 cannot be overemphasized; as was shown very clearly by Jeener,^^'^^ the 

 relationships between PNA content and protein synthesis are very different 

 in the case of the flagellate Polytcmella coeca, whether one is dealing with a 

 continuous culture (in the exponential phase of growth) or not : if one com- 

 pares cells during the various stages of growth of a culture, there is no 

 linear relationship between the quantity of PNA per milligram protein 

 nitrogen and the rate of protein synthesis (Jeener^^). If, on the other hand, 

 continuous cultures are used under conditions where growth maintains 

 itself during long periods at a constant rate which can be varied at will 

 between wide limits, a strict relationship is found between the rate of pro- 

 tein synthesis and the quantity of PNA in excess of a constant basal figure 

 always present in the cells. Thus the close relationship between the quantity 

 of PNA present and protein synthesis exists only for systems in the steady 

 state; when the physiological conditions of the cells are changing rapidly, 

 for instance at the end of the lag and logarithmic phases of growth, no 

 such simple correlation can be found (Jeener^*) . 



These conclusions are by no means unexpected if one recalls the afore- 

 mentioned cytochemical findings of Malmgren and Hed^n:^^ using Caspers- 

 son's ultraviolet absorption technique, they found the absorption to be very 

 low in an 18-hr. agar culture of B. cereus. There is a moderate increase in 

 the absorption due to PNA during the lag phase. However, during the 

 logarithmic phase of growth the ultraviolet absorption is very intense and 

 the PNA content then reaches its peak. It finally falls to the original low 

 level and all divisions cease at the same time — cell division is thus, in this 

 case, very clearly and closely related to the PNA content. 



c. Additional Evidence 



Beside cytochemical and quantitative evidence indicating a role of PNA 

 in protein synthesis additional evidence, although of a more circumstantial 

 nature, should be briefly mentioned. 



^* H. B. Levy, E. T. Skutch, and A. L. Schade, Arch. Biochem. 24, 198 (1949). 



96 Y. Miura and Y. Nakamura, Bull. soc. chim. biol. 33, 1409 (1951). 



98 Y. Miura, Y. Nakamura, and H. Matsudaira, Bull. soc. chim. biol. 33, 1577 (1951). 



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



9* R. Jeener, Arch. Biochem. and Biophys. 43, 381 (1953). 



