Transcriptional slippage in the positive-sense RNA virus family Potyviridae

Abstract

The family Potyviridae encompasses ~30% of plant viruses and is responsible for significant economic losses worldwide. Recently, a small overlapping coding sequence, termed pipo, was found to be conserved in the genomes of all potyvirids. PIPO is expressed as part of a frameshift protein, P3N‐PIPO, which is essential for virus cell‐to‐cell movement. However, the frameshift expression mechanism has hitherto remained unknown. Here, we demonstrate that transcriptional slippage, specific to the viral RNA polymerase, results in a population of transcripts with an additional “A” inserted within a highly conserved GAAAAAA sequence, thus enabling expression of P3N‐PIPO. The slippage efficiency is ~2% in Turnip mosaic virus and slippage is inhibited by mutations in the GAAAAAA sequence. While utilization of transcriptional slippage is well known in negative‐sense RNA viruses such as Ebola, mumps and measles, to our knowledge this is the first report of its widespread utilization for gene expression in positive‐sense RNA viruses.