Specificity of ribonucleic acid chain initiation by bacteriophage T3 induced ribonucleic acid polymerase

Further studies on the specificity of RNA chain initiation by T3 induced RNA polymerase were carried out. The nucleotide penultimate to the 5' guanosine triphosphate end of RNA chains was determined. GMP is found exclusively in this second position of a growing RNA chain. The ³²PP(i) exchange reaction with ribonucleoside triphosphates catalyzed by T3 RNA polymerase on native T3 DNA template was also studied as a measure of the initiation reaction. It has been found that T3 RNA polymerase carries out an active exchange of ³²PP(i) into ATP in the presence of GTP. In contrast, in reactions containing GTP alone or with GTP + CTP or GTP + UTP, an active exchange of ³²PP(i) only occurs into GTP. No ³²PP(i) incorporation was detected into either CTP or UTP. These results suggest that all RNA chains formed in T3 RNA polymerase reaction directed by native T3 DNA as template contain the sequence pppGp(Gp)n(Ap)m... (where n and m = 1 or more) at the 5' terminus, indicating a high degree of specificity of initiation of RNA chains in this system.