52 



MAURICE J. BESSMAN 



preparations. As mentioned previously, Kornberg (1960) has pointed out 

 that some preparations of E. coli polymerase prefer denatured DNA 

 preparations as primers. Bollum has suggested that the reason a dena- 

 tured ])rinu'r re(iuirement is more easily demonstrated with calf thymus 

 polymerase than witii the corresponding enzyme from E. coli is that 

 thymus glands contain much less nuclease than E. coli and thus poly- 

 merase preparations prepared from this tissue have relatively less 

 nuclease activities earlier in the purification procedure. As mentioned 



TABLE XXVI 

 Comparison of Puimer-Product Base Ratios with Calf Thymus Polymerase" 



" From Bollum (1959b). 



before (Section IV,E) the presence of nuclease in the enzyme prepara- 

 tions could convert non-priming (native) DNA to priming (denatured) 

 DNA thus obscuring a requirement for denatured DNA preparations. 

 The implication in all these studies is that the natural primer for DNA 

 synthesis is single-stranded DNA. As in the E. coli system, the single- 

 stranded DNA from bacteriophage ^X174 is an excellent primer for 

 calf thymus polymerase and requires no prior heating to manifest its 

 activity. Other means of denaturing DNA such as acid or alkali treat- 

 ment are also effective in converting non-priming to priming DNA 

 (Table XXVI). 



Calf thymus and E. coli polymerase differ in that no "unprimed" 

 reactions have yet been demonstrated with the mammalian enzyme. 

 Calf thymus polymerase does not catalyze the unprimed synthesis of the 



