Unusual priming mechanism of RNA-directed DNA synthesis in copia retrovirus-like particles of Drosophila

Drosophila cells contain virus-like particles (VLPs) containing 5 kilobases (kb) of RNA (VLP H-RNA) homologous to the transposable element copia ¹. The identity between VLP H-RNA and copia DNA has previously been confirmed at the nucleotide sequence level² and reverse transcriptase activity is also detected in the VLPs¹. These results suggest that VLPs and copia are derivatives of viral particle and provirus forms, respectively, of the copia retrovirus-like particle. If the copia retrovirus-like particle replicates by a mechanism similar to the mechanism of vertebrate retroviral replication, a cellular transfer RNA would prime synthesis of the first DNA strand. We show that this is indeed so but that copia retrovirus-like particle has a novel type of priming mechanism; the first DNA extension does not start from the 3′ end of a tRNA, but from an internal site (two nucleotides after the anticodon loop) of the Drosophila initiator methionine tRNA (tRNA_i ^Met)