We have recently shown that a newly isolated avian sarcoma virus, UR2, is defective in replication and contains no sequences homologous to the src gene of Rous sarcoma virus. In this study, we analyzed the genetic structure and transforming sequence of UR2 by oligonucleotide fingerprinting. The sizes of the genomic RNAs of UR2 and its associated helper virus, UR2AV, were determined to be 24S and 35S, respectively, by sucrose gradient sedimentation. The molecular weight of the 24S UR2 genomic RNA was estimated to be 1.1 x 106, corresponding to 3,300 nucleotides, by gel electrophoresis under the native and denatured conditions. RNase T1 oligonucleotide mapping indicated that UR2 RNA contains seven unique oligonucleotides in the middle of the genome and shares eight 5'-and six 3'-terminal oligonucleotides with UR2AV RNA. From these data, we estimated that UR2 RNA contains a unique sequence of about 1.2 kilobases in the middle of the genome, and contains 1.4 and 0.7 kilobases of sequences shared with UR2AV RNA at the 5' and 3' ends, respectively. Partial sequence analysis of the UR2-specific oligonucleotides by RNase A digestion revealed that there are no homologous counterparts to these oligonucleotides in the RNAs of other avian sarcoma and acute leukemia viruses studied to date. UR2-transformed non-virus-producing cells contain a single 24S viral RNA which is most likely the message coding for the transforming protein of UR2. On the basis of the uniqueness of the transforming sequence, we concluded that UR2 is a new member of the defective avian sarcoma viruses.

Rous sarcoma virus-29 (RSV-29) is the strain of RSV that has the least number of passages beyond its isolation from chicken tumor no. 1 among all current strains of RSV. Biological characterization indicated that it was replication defective. RNA analysis of nonproducer clones of RSV-29-infected chicken embryonic fibroblasts showed the presence of a subgenomic message of 2.6 kilobases containing src and a genomic RNA of 7.7 kilobases that contains gag, pol, and src, but not env. The src-containing EcoRI fragment of RSV-29 proviral DNA was molecularly cloned. Sequence analysis of the regions flanking src revealed that the env gene was completely deleted in RSV-29 and that the sequence across the deletion was exactly the same as the Bryan high-titer strain of RSV. The sequence immediately 3' to src in RSV-29 was closely related to that of the Prague strain of RSV. The fact that the strain of RSV which has the minimal number of passages beyond its isolation is replication defective supports the hypothesis of Lerner and Hanafusa (J. Virol. 49:549-556, 1984) that the original RSV is a defective transforming virus. This defective transforming virus is postulated to be the precursor to other defective RSVs like the Bryan high-titer strain and to nondefective RSVs like the Prague strain. The particular clone of RSV-29 that we studied also had a short stretch of sequence duplication at the 3' end of the pol gene, which was presumably created by an error of reverse transcription.