Analysls of aberrant ribosomal DNA (rDNA) repeats of Bombyx mori resulted In the discovery of a 4.8 kllobase retrotransposable element, Pao. Approximately 40 copies of Pao are present In the genome with most located outside the rDNA units. The complete sequence of one Pao element and partial sequence of four other copies Indicated that Pao encodes an 1158 amino acid open-reading frame (ORF). Located within this ORF are domains with sequence similarity to retroviral gag genes, aspartic protease and reverse transcriptase. RNase H and Integrase domains were not Identified suggesting that the cloned copies were not full-length elements. Pao elements contain long terminal repeats (LTRs) with a central region composed of variable numbers of 46 bp tandem repeats. The variable region appears to correspond to the R region of retroviral LTRs, the region responsible for strand transfer during reverse transcription. Based on a sequence analysls of its reverse transcriptase domain, Pao is most similar to TAS of Ascaris lumbricoldes. Pao and TAS represent a subgroup of LTR retrotransposons distinct from the Copla-Ty1 and Gypsy-Ty3 subgroups.

Author notes

+Present address: PO Box 100, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA