Self-processing 2A-polyproteins--a system for co-ordinate expression of multiple proteins in transgenic plants.

Achieving co-ordinate, high-level and stable expression of multiple transgenes in plants is currently difficult. Expression levels are notoriously variable and influenced by factors that act independently on transgenes at different genetic loci. Instability of expression due to loss, re-arrangement or silencing of transgenes may occur, and is exacerbated by increasing numbers of transgenic loci and repeated use of homologous sequences. Even linking two or more genes within a T-DNA does not necessarily result in co-ordinate expression. Linking proteins in a single open reading frame--a polyprotein--is a strategy for co-ordinate expression used by many viruses. After translation, polyproteins are processed into constituent polypeptides, usually by proteinases encoded within the polyprotein itself. However, in foot-and-mouth disease virus (FMDV), a sequence (2A) of just 16-20 amino acids appears to have the unique capability to mediate cleavage at its own C-terminus by an apparently enzyme-independent, novel type of reaction. This sequence can also mediate cleavage in a heterologous protein context in a range of eukaryotic expression systems. We have constructed a plasmid in which the 2A sequence is inserted between the reporter genes chloramphenicol acetyltransferase (CAT) and beta-glucuronidase (GUS), maintaining a single open reading frame. Here we report that expression of this construct in wheatgerm lysate and transgenic plants results in efficient cleavage of the polyprotein and co-ordinate expression of active CAT and GUS. Self-processing polyproteins using the FMDV 2A sequence could therefore provide a system for ensuring co-ordinated, stable expression of multiple introduced proteins in plant cells.