174 KARL C. LARK 



man <■/ <//.. H)()()i. 1 lowcvci', as cell .^i-owili pio^ix'sso, the enzyme ac- 

 tivities arc ay;aiii ledured, thymidine diphospliokinase ])v'w^ lust more 

 rapidly (80''/ of il> actixity disappearm.^ in 24 hours). Addition of thy- 

 midine to the medium appears to stabilize the enzymes ( W'eissman ct al., 

 1960; Bojarski and lliatt, 19()()l. A similar seciuence of event> has been 

 observed in regenerating liver. Such loss of enzymatic activity does not 

 appear to be uni(iue to the nucleotide kinases since similar findings have 

 been observed with glutamyl transferase (Fottrell and i*aul, 19()1). 

 Similarly, phosphoiylating activity which has appeared in the micro- 

 spores of Tnllium shortly before DNA synthesis also disappears again 

 within a short time (Hotta and Stern, 1961b). Thus, loss of enzymatic 

 activity during the course of the DNA cycle could occur.- However, in 

 no case has the loss of acti\ity been sufficiently rapid to account for 

 cessation of DNA synthesis within 4-10 hours (by the end of the syn- 

 thetic period) nor can the observed dependence of enzyme stability on 

 the medium composition and/or the conditions of growth be reconciled 

 with a precise control mechanism which halts DNA synthesis after an 

 exact duplication of the intracellular DNA content. Thus, although 

 enzj'me biosynthesis ajipears to exei'cise a negative tyjic of conti'ol over 

 the initiation of DNA biosynthesis, as in the case of regenerating liver, 

 such a mechanism cannot account for the degree of control observed. 



A more important influence in the regularly dividing cell may be the 

 synthesis of proteins or histones necessary to stabilize the structure^ of 

 the new DNA. It is quite possible that DNA synthesis is linked to the 

 formation of such proteins by a mechanism whereby synthesis of the 

 former cannot proceed without simultaneous or piior syntliesis of the 

 latter (see Section TV). 



To answer these (luestions, a more general approach to the role of 

 protein synthesis in regulating DNA formation has been devised by 

 several workers who have studied the effects of inhibiting protein synthe- 

 sis on subse(iuent DNA synthesis. Two tyi)es of inhibition have l)een 

 cmjiloyecl: {(i) starvation for an essential amino acid, and ih) addition 

 of an agent which inhibits protein synthesis. 



It has been generally observed that bacteria starved for amino acids 

 required for growth synthesize a small amount of DNA, 50-150% of the 

 pre-existing amount, after which all further synthesis ceases (Pardee 

 and Prestidge, 1956; Earner and Cohen, 1957; Goldstein et al, 1959; 

 Okazaki and Okazaki, 1959; Gros and Oros, 1958; Billen, 1961; Maal0e 

 and Hanawalt, 19611. In such starved cells, all demonstrable protein 



"Recently the i)os.sil)ility ha.s been raised lliat tlie observed lark of enzyme 

 activity in system.s .sueh as regenerating liver may he due to the conversion of 

 intracellular enzyme from a soluble into a particulate form (Bianclii d dJ., 1061). 



