42 I. LESLIE 



tures of E. coli,'^°^ and during the rapid gro\\'th phase of Micrococcus pyro- 

 genes^^'^; following intensive fermentation, the PNA concentration per unit 

 dry weight is increased in baker's yeast.^"* 



A rise in the PXA/DNA ratio occurs during the early growth stages of 

 Staph, aureus,^^^ in E. coli,^^^ and over the first 3 hr. gro^vth of Salmonella 

 and Shigella cultures.'^" In these last, the PNA per unit PN increased w^hile 

 the DNA per unit PN decreased over the first part of the growth phase. In 

 the lag phase of Staph, muscae cultures, Price-^^ reports a 60 % increase in 

 PNA and protein content, and a 50 % increase in dry Aveight, without any 

 change in cell count. DNA synthesis began only 45 min. after that of PNA 

 and PN (cf. chick heart explants, Section V). 



Caldwell and Hinshelwood^^^ have carried out an interesting investiga- 

 tion into the actual content per cell during the growth of cultures of B. 

 lactis aerogenes. Although the cell nitrogen content varies between 50 and 

 220 mg. N per 10^^ cells, the DNA per cell remained constant at about 2 

 mg. DNA-P per 10^^ cells. The PNA content fluctuated between values of 

 3 and 32 mg. PNA-P per 10'- cells. Colchicine did not alter the amount of 

 DNA per cell, but it did increase the PNA/DNA ratio. During the normal 

 growth of cultures,-'^ the PNA-P per unit cell N and per cell rose and main- 

 tained a roughly linear relationship to the reciprocal of the mean generation 

 time (i.e. the "growth rate")- Caldwell and Hinshelwood-'"* have also pro- 

 posed that "towards the end of the growth cycle when the medium becomes 

 depleted of phosphorus, there is in fact an extensive conversion of PNA to 

 DNA." In growing cultures of B. lactis aerogenes, the PNA-P and DNA-P 

 account for 61 % and 18 %, respectively, of the total P content of the cells. 



Complications can arise when the standard methods for determining 

 PNA and DNA are applied to bacterial cells.^^' ^' In cultures of Micrococcus 

 pyogenes, PNA and DNA account for 29 % and 11 % of the total P.^^ Sher- 

 ratt and Thomas^' report that the "bound DNA" Cinsoluble in alkaline 

 digest III, Table I) has 22 % excess P, while the PNA fraction contains 15 % 

 excess P. All these fractions reached their maximal amount per cell during 

 the logarithmic phase of growth. 



According to Mitchell and Moyle,-°^ the PNA concentration in cells of 

 Micrococcus pyogenes appears to control the rate of growth. Addition of 



206 M. L. Morse and C. E. Carter, /. Bacteriol. 58, 317 (1949). 



207 P. Mitchell and J. Moyle, /. Gen. Microbiol. 5, 421 (1951). 



208 H. von Euler and L. Hahn, Arkiv Kemi, Mineral. Geol. 25A, 10pp. (1948). 



"9 C. A. Fish, I. Asimov, and B. S. Walker, Proc. Soc. Exptl. Biol. Med. 75, 774 (1950). 



2'o T. Brechbuhler, Bull. soc. chim. hiol. 32, 952 (1950). 



211 W. H. Price, J. Gen. Physiol. 35, 741 (1952). 



2>2 P. C. Caldwell and Sir Cyril Hinshelwood, /. Chem. Soc. 1950, 1415. 



213 p. C. Caldwell, E. L. Mackor, and Sir Cyril Hinshelwood, /. Chem. Soc. 1950, 



3151. 

 21" P. C. Caldwell and Sir Cyril Hinshelwood, J. Chem. Soc. 1950, 3156. 



