Vesicular stomatitis virus and its defective interfering particles exhibit in vitro transcriptional and replicative competition for purified L-NS polymerase molecules.

We have quantitated replication of the RNA genomes of vesicular stomatitis virus (VSV) and its defective interfering (DI) particles in a BHK21 cell-free system into which nucleocapsids were introduced in varying amounts and ratios, with or without addition of purified virus polymerase components. The quantitative transcriptional and replicative competition observed in vitro between virus and DI genomes resembled DI particle interference observed in vivo in infected cells. The effects of an added polymerase protein (L-NS) complex from purified virions showed that this competition varies with polymerase availability. When DI nucleocapsids were added in small amounts, addition of L-NS polymerase protein complex stimulated a linear increase in viral mRNA transcription until the viral templates' transcription capacity became saturated; then there was a reproducible sudden switch toward RNA replication (mainly of DI genomes). Purified L or NS proteins added separately produced different effects than the L-NS complex. These findings support earlier evidence for replicative competition as the mechanism of DI particle interference with standard virus, and suggest that the major competition is for limiting amounts of L-NS molecules involved in transcription and replication, and in facilitation of encapsidation.