An avian retrovirus uses canonical expression and processing mechanisms to generate viral microRNA

To date, the vast majority of virus-encoded miRNAs are derived from polymerase II transcripts encoded by DNA viruses. Recent demonstration that the bovine leukemia virus, a retrovirus, uses RNA polymerase III to directly transcribe the pre-miRNA hairpins to generate viral miRNAs further supports the common notion that the canonical pathway of miRNA biogenesis doesn't exist commonly among RNA viruses. Here, we show that the E (XSR) element of avian leukosis virus subgroup J (ALV-J), a member of avian retrovirus, encodes a novel miRNA, designated as E (XSR) miRNA, using the canonical miRNA biogenesis pathway. Detection of novel microRNA species derived from E (XSR) element, a 148-nucleotide non-coding RNA with hairpin structure, showed that E (XSR) element has the potential to function as microRNA primary transcript, demonstrating a hitherto unknown function with possible roles in myeloid leukosis associated with ALV-J.