An expression vector comprising a promoter capable of directing expression of a coding sequence in an angiosperm cell, and a 3' untranslated region of a rice .alpha.-amylase gene, wherein, after the coding sequence is inserted into the vector, the 3' untranslated region and the coding sequence are transcribed into a single mRNA. Also featured are methods of producing a polypeptide in angiosperm cells using such a vector.