418 F. GROS 



varies according to the physiological state of the bacteria. When cultivated 

 in a low phosphate content medium, E. coli becomes uninucleated. 63 



Especially since the discovery of bacterial transformations, 64 bacterial 

 DNA has been thoroughly studied with respect to its chemical 65 and 

 physicochemical properties, 66 and its biological activity, 67-71 but little is 

 known about the nature of the bacterial protein associated with the DNA. 42 



Contrary to what seems to be the rule for other kinds of cells which syn- 

 thesize DNA only during a certain period of the mitotic cycle (late inter- 

 phase 7274 ), it appears that bacteria synthesize DNA in a continuous man- 

 ner. This interpretation can be derived from the rate of decay of P 32 in 

 unsynchronized cultures of bacteria labeled for a very short time, 75 or from 

 the rate of uptake of tritiated thymidine by individual bacteria. 76 



5. The Cell Wall and the Cytoplasmic Membrane 



Consideration of the chemical structure and of the mode of biosynthesis of the 

 cell wall may be of interest in a study of incorporation of radioactive amino acids 

 into bacteria, since, in some cases, the observed incorporation may represent the 

 exclusive synthesis of the mucopolypeptides of the cell wall rather than the formation 

 of true cytoplasmic protein. 



In gram-negative bacteria the cell wall is composed of two layers. The outside 

 layer is a lipoprotein, and the inner is a complex of lipopolysaccharides with a muco- 

 polypeptide. In the peptide moiety, diaminopimelic acid, glutamic acid, and alanine 

 are always present. In addition, lysine, serine, and glycine have sometimes been 

 found. 7779 The cell wall of gram-positive bacteria is simpler and contains no lipo- 

 protein, but a mucopolypeptide containing glutamic acid and alanine residues to- 

 gether with either diaminopimelic acid or lysine. The bacterial membrane can be 

 prepared by osmotic lysis of protoplasts, that is, the bacterial cell stripped of its 



63 E. McFall, A. B. Pardee, and G. S. Stent, Biochim. et Biophys. Ada 27, 282 (1958). 



64 0. T. Avery, C. M. MacLeod, and McCarty, ./. Exptl. Med. 89, 137 (1944). 



66 E. Chargatf, in "The Nucleic Acids" (E. Chargaff and J. N. Davidson, eds.), 

 Vll. I, p. 307. Academic Press, New York, 1955. 



66 M. Meselson and F. \Y. Stahl, Proc. Sail. Acad. Sci. U. S. 44, 671 (1958). 



67 R. D. Hotchkiss, Cold Spring Harbor. Symposia Quant. Biol. 16, 457 (1951). 



68 R. D. Hotchkiss, in "The Nucleic Acids" (E. Chargaff and J. N. Davidson, eds.), 

 Vol. II, Chapter 27. Academic Press, New York, 1955. 



69 R. D. Hotchkiss, Harvey Lectures 49, 124 (1955). 



70 H. Ephrussi-Taylor, Cold Spring Harbor Symposia Quant. Biol. 16, 445 (1951 ). 



71 H. Ephrussi-Taylor, Exptl. Cell Research 2, 589 (1951). 



72 J. Pasteels and L. Lison, Arch. biol. (Liege) 61, 445 (1950). 



73 H. H. Swift, Physiol. Zool. 23, 169 (1950). 



74 J. H. Taylor and R. I). MeMaster, Chromosoma 6, 489 (1954). 



75 E. McFall and G. S. Stent, quoted in 178. 



76 M. Schaechter, M. W. Bentzon, and O. Maal0e, Nature 183, 1207 (1959). 



77 E. Work, Biochem. J. 49, 17 (1951). 



78 E.Work,A'a7«>-el79,841 (1957). 



79 M. R. J. Salton, in "Bacterial Anatomy," p. 81. Cambridge Univ. Press, London 

 and New York, 1956. 



