398 POSTSCRIPT 



In 1960, independently in three laboratories (llurvvitz et ai, Weiss et 

 ai, Stevens), an enzyme called RNA polymerase was described which 

 catalv/.es the DNA-dependent synthesis of a complementary RNA. The 

 base composition of the newly synthesized RNA is rigorously determined 

 by the DNA used as primer, in a manner wholly analogous to the replica- 

 tion of DNA catalyzed by DNA polymerase (p. 186). The four ribo- 

 nucleoside triphosphates are required for RNA synthesis and the poly- 

 merization leads to the release of stoichiometric amounts of pyrophos- 

 phate. The DNA acts as a primer or catalyst in the sense that the RNA 

 product formed greatly exceeds the input DNA in quantity; but DNA is 

 also the template since the RNA formed is complementary to it. 



There is as yet no proof that the RNA polymerase .system functions in 

 the cell to make complementary RNA, but the existing evidence supports 

 this view. Further indirect evidence comes from the demonstration by 

 Hall and Spiegelman in 1961 of the presence in phage-infected and in 

 uninfected bacteria of DNA-RNA hybrid molecules, presumed to have 

 been isolated while in the process of RNA synthesis on the DNA template. 

 They also showed that artificial hybrid molecules could be formed from 

 DNA and a special RNA fraction; the hybrid-forming ability was attrib- 

 uted to a close complementarity in base sequence between the DNA and 

 RNA. Hybridization was proposed as a criterion for identification of a 

 complementary RNA of biological origin corresponding to the in vitro 

 product of RNA polymerase activity. 



A method has been developed by Bautz and Hall for the isolation of 

 T4 messenger RNA corresponding to a particular genetic segment, using 

 the mutant phage T4 carrying a deletion in the r // region (p. 169). The 

 method involves formation of DNA-RNA hybrid molecules between T4- 

 DNA and the RNA formed after infection of E. coli with T4. Messenger 

 RNA corresponding to the deleted region of DNA will be formed when 

 wild-type T4 is used but not when the mutant strain is used for infection. 

 Formation of hybrid molecules, in this experiment, occurs on a chromato- 

 graphic column to which DNA is adsorbed first, and RNA then added. 

 When both DNA and RNA come from the wild-type system, the comple- 

 mentary RNA of the deleted region is adsorbed onto the column. How- 

 ever, when the DNA comes from the mutant strain, and the RNA from 

 cells infected with the wild-type, then the RNA molecules corresponding 

 to the deleted region are not hybridized on the column, and are thereby 

 separated from the rest of the RNA. The fact that this experiment works 

 is in itself a substantial confirmation of the existence of messenger RNA, 

 as well as providing a powerful new analytical tool. 



