FIG. 1.

Genomic structures of endogenous and exogenous type D retroviruses of sheep. Premature stop codons are indicated by a vertical bar underlined by an asterisk. For convenience, the gag open reading frame has been fixed in the same reading frame of all sequences shown. The numbered bar at the bottom indicates distances in kilobases. The exogenous JSRV and ENTV show the canonical retroviral gag, pro, pol, and env with pro in a different open reading frame from pol, the same for all type D and B retroviruses. An additional open reading frame (orf-x) overlapping pol is present in JSRV but is interrupted by two stop codons in ENTV (8). enJS56A1 is the only one of the three endogenous proviruses cloned in this study to maintain full (or nearly full) open reading frames in all structural genes. enJS59A1 has premature stop codons in gag and pol and a major deletion in env. enJS5F16 has a deletion in pol. Different peptide sequences at the 3′ end of the pol gene in enJS56A1 due to a frameshift are indicated by cross-hatching. The LTRs are indicated by solid boxes.