Capped poly(A) leaders of variable lengths at the 5' ends of vaccinia virus late mRNAs

Evidence for capped poly(A) leaders of variable lengths located immediately upstream of the translation initiation codon was obtained by direct analyses of a major late mRNA species. A decapping-recapping method was used to specifically substitute a radioactively labeled phosphate for an unlabeled one within the cap structure. RNase H-susceptible sites were made by hybridizing synthetic oligodeoxyribonucleotides to the mRNA encoding a late major structural protein of 11 kilodaltons. Sequences of the type m⁷G(5')pppAmp (Ap)(n)UpG..., where n varies from a few to more than 40 nucleotides, were deduced by analysis of the length and sequence of RNase H, RNase, T₁, and RNase U2 digestion products.