A protein expression system derived from the RNA-2 segment of the Comoviridae virus corresponding to cowpea mosaic virus (CPMV). The expression system comprises: (a) An expression enhancer element that comprises a part of the RNA-2 genome segment in which the initiation site at position 161 of the wild-type sequence has been mutated. The RNA-2 genome segment encodes two carboxy coterminal protein through two different translation sites located in the same triplet reading frame and (b) A heterologous sequence that can either facilitate the insertion of a gene of interest or encodes for a gene of interest and is located downstream of the mutated initiation site in the enhancer sequence and optionally (c) A 3 UTR. The levels of protein expression can be increased 20-30 fold using the expression system compared to a vector with an intact start codon at position 161. The expression system allows for production of high levels of the expression product without the need for viral replication and reduces necrosis in transformed plants.
