The polymerase slips and PIPO exists.

Plant viruses that contain plus-sensed single-stranded RNA genomes are highly abundant in nature. As the equivalents of large mRNAs, these viral genomes utilize a wide variety of gene expression strategies for the production of their encoded proteins. The potyviruses, which are among the most agriculturally important members in this category, contain a single large open reading frame (ORF) coding for a polyprotein that is processed into functional units. For many years, the products derived from the full-length polyprotein were thought to be the only functional viral proteins. However, this notion was dispelled when an additional essential viral ORF, PIPO, was discovered encoded in an alternative reading frame. Since then, the PIPO protein— P3N-PIPO, which mediates virus movement in plants—has been intensively studied, but its mode of expression remained elusive until now. Two articles, one in this issue of EMBO Reports, now report that slippage of the viral polymerase during viral genome replication is responsible for shifting PIPO into a translated reading frame, thereby allowing for production of P3N-PIPO [1,2]. This mechanism of gene expression represents a novel strategy for plant viruses.