Vaccinia virus mRNAs carry the cap structure mGpppAm- or mGpppGm- at the 5′-terminus, which is synthesized by a series of RNA polymerase and capping enzymes contained in the virus particle. The process of the cap formation at the 5′-terminus of mRNA was studied using an system under similar conditions to those of vaccinia virus multiplication in its host cell. After adding a methyl-group donor, methyl-H--adenosylmethionine, the oligonucleotides, which were the synthesized 5′-terminal part of mRNA, were isolated from the RNA-synthesizing virion at appropriate time intervals, and were analysed. The 5′-5′ confronting nucleotides with 2′--methylation, GpppAm and GpppGm, were found with the completed cap structure, mGpppAm and mGpppGm. The confronting nucleotides with only 7-methyl guanine as a methylated component, mGpppA and mGpppG, were not detected at any incubation time, and it was concluded that methylation at the 2′-position of the 5′-terminal purine nucleoside of mRNA precedes methylation of the 7-position of the blocking guanosine. This result is different from that obtained using the enzymes isolated from vaccinia virus (Moss , 1976) and also from the results obtained using other kinds of virus particles, which carry RNA polymerase and capping enzymes. These differences may be due to the specific organization of a series of capping enzymes and RNA polymerase in each virus particle.


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