Synthesis of capped and uncapped methylated oligonucleotides by the virion transcriptase of spring viremia of carp virus, a rhabdovirus.

Capped and uncapped methylated oligonucleotides, five and six nucleotides in length, were synthesized in vitro in spring viremia of carp virus transcription reaction mixtures containing ATP + CTP, ATP + CTP + GTP, or ATP + CTP + UTP, but not in any other combinations of two or three ribonucleoside triphosphates. The oligonucleotides that have been characterized are consistent with the structures: m7GpppAmpNpCpNpN, GpppAmpNpCpNpN, pppAmpNpCpNpN, and ppAmpNpCpNpN--i.e., similar to those of the termini of transcripts made in complete reaction mixtures. Because both capped and uncapped methylated oligonucleotides were synthesized, it can be concluded that methylation of the penultimate nucleotide can precede capping and methylation of the capping nucleotide. Our results also indicate that capping and methylation are processes that can take place prior to mRNA chain completion.