This is the accepted manuscript. It is currently embargoed pending publication. ; 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. ; Work in the AEF lab was funded by grants from the Wellcome Trust [088789], [106207] and Biotechnology and Biological Research Council (BBSRC) [BB/J007072/1], [BB/J015652/1]. Work in the JPC lab was funded by BBSRC grants [BB/J015652/1], [BB/J011762/1]. BYWC was supported by a Sir Henry Wellcome Postdoctoral Fellowship [096082] and an EMBL long-term postdoctoral fellowship.