AN ENV-LIKE PROTEIN ENCODED BY A DROSOPHILA RETROELEMENT - EVIDENCE THAT GYPSY IS AN INFECTIOUS RETROVIRUS

The gypsy element of Drosophila differs from most LTR retrotransposons in containing a third open reading frame that resembles retroviral en v genes. The protein encoded by ORF3 is glycosylated and processed, li ke all retroviral envelope proteins. The protein is expressed at high levels in fly strains in which gypsy elements are active. In these str ains the protein is found primarily in viral particles. When larvae of fly strains in which gypsy is normally inactive are exposed to sucros e gradient fractions containing these particles, a high level of gypsy insertion activity is observed in their progeny. Thus, gypsy has the expected properties of an insect retrovirus.