The retrotransposon copia controls the relative levels of its gene products post-transcriptionally by differential expression from its two major mRNAs.

All retroviruses and retrotransposons studied to date regulate the relative levels of gag and pol/int gene products post-transcriptionally from a single mRNA. In these genetic elements the production of protein encoded by the pol and int genes is attenuated by a translational stop or frameshift in the reading frame preceding their coding regions in the mRNA. We show here that the Drosophila retrotransposon copia also produces lower amounts of gene products from its int/pol region than gag region but this is achieved by a mechanism which is novel for this class of genetic element. We show by the use of gene fusion constructs that the subgenomic 2 kilobase copia RNA, encoding gag products, is expressed as protein in cultured cells at least ten-fold more efficiently than the full genome length RNA, which additionally contains the pol and int open reading frames.