FIG. 6.

Paramyxovirus genome length correction. The viral RNAs are shown as horizontal lines; the filled symbols at the extremities indicate 3′ ends. The direction of RNA synthesis is indicated by arrows. Unnatural nonhexamer-length genomes (e.g., 6n+5) can be introduced into cells via T7 RNA polymerase expression vectors. Although they replicate poorly, their constant production by the bacteriophage polymerase ensures that some antigenome synthesis occurs. Purine insertions take place during antigenome synthesis as well as during mRNA synthesis, and those which are now of hexamer length replicate more efficiently and accumulate, leading to correction of the genome length.