128 H. GOBIND KHORAXA 



priming capacity of different polynucleotides and the nucleoside diphos- 

 phates being polymerized. 38, 40 The synthesis of homopolymers could be 

 primed by the addition of the corresponding homopolymers; thus, for exam- 

 ple, the polymerization of ADP was primed by polyadenylic acid. The syn- 

 thesis of polyuridylic acid as well as the formation of the mixed polymers 

 of adenylic, guanylic, cytidylic, and uridylic acids was primed somewhat by 

 yeast RNA, and more effectively by preformed mixed polymer or Azotobac- 

 ter RNA. However, the synthesis of polyuridylic acid was inhibited by the 

 addition of polyadenylic acid and, correspondingly, the synthesis of poly- 

 adenylic acid was inhibited by polyuridylic acid. Again, the synthesis of 

 polycytidylic acid was primed only by polycytidylic acid but was strongly 

 inhibited by polyadenylic acid, polyuridylic acid, RNA, or the mixed 

 synthetic polymer of the four nucleotides. On the other hand, polycytidylic 

 acid primed not only its own synthesis but also that of all other polynuc- 

 leotides, including polyadenylic acid, polyuridylic acid, and the mixed 

 polymer of all the four nucleotides. 



(2) Oligonucleotides as Primers. The extensive and definitive studies of 

 Heppel and co-workers 36, 4141a on the utilization of oligonucleotides as primers 

 have not only thrown light on the mode of action of the enzyme but have 

 also made available several new types of polynucleotides in which the end 

 groups and environment near one terminus are known with certainty. The 

 main results may be briefly summarized. 



Homologous series of oligonucleotides (pApA, pApApA, pApApApA) as 

 well as those bearing no 5'-phosphomonoester end groups (ApA, ApU, 

 ApApA) serve as primers and are effectively incorporated into the poly- 

 mers synthesized. All evidence points to the initiation of the synthesis of 

 polynucleotide chains onto the ends of these primers bearing the 3 '-hydroxy 1 

 group. 



The minimum sized primer is a dinucleoside phosphate containing a 

 C 5 '-C 3 ' diester bond. Thus ApA and ApU serve as primers, although not as 

 effectively as the higher homologs. The concentrations required for maximal 

 stimulation of the rate of synthesis decreases with increase in chain length 

 of the primer. Approximate concentrations for such rates are 2.4 X 10~ 3 M, 

 4.2 X 10" 4 M, and 1.7 X 10" 4 M, respectively, for pApA, pApApA, and 

 pApApApA. Similar results have been obtained with the corresponding 

 compounds lacking 5'-phosphomonoester groups. The rates using optimal 

 concentrations of the homologous series of oligonucleotides were the same 

 in the synthesis of polyadenylic acid as well as polyuridylic acid, although 

 the optimal rates using the two nucleoside diphosphates differed. 



Unlike the results described above for priming by high molecular weight 

 polynucleotides, no evidence of specificity in the priming by oligonucleotides 

 was obtained. Thus pApApA and related compounds served as primers and 



