Spleen focus-forming Friend virus: identification of genomic RNA and its relationship to helper virus RNA

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Abstract
The genome of the defective, murine spleen focus-forming Friend virus (SFFV) was identified as a 50S RNA complex consisting of 32S RNA monomers. Electrophoretic mobility and the MW of unique RNase T1-resistant oligonucleotides (T1-oligonucleotides) indicated that the 32S RNA had a complexity of about 7.4 kilobases. Hybridization with DNA complementary to Friend murine leukemia virus (Fr-MLV) distinguished two sets of nucleotide sequences in 32S SFFV RNA, 74% which were Fr-MLV related and 26% which were SFFV specific. By the same method, SFFV RNA was 48% related to Moloney MLV. Large T1-oligonucleotides (23) of SFFV RNA and 43 of Fr-MLV RNA were resolved. On the basis of the relationship between SFFV and Fr-MLV RNA, the 23 SFFV oligonucleotides fell into 4 classes: i, 7 which had homologous equivalents in Fr-MLV RNA; ii, 6 more which could be isolated from SFFV RNA-Fr-MLV c[complementary]DNA hybrids treated with RNases A and T1; iii, 8 more which were isolated from hybrids treated with RNase T1; and iv, 2 which did not have Fr-MLV-related counterparts. Surprisingly, the 2 iv class oligonucleotides had homologous counterparts in the RNA of 6 amphotropic MLV including mink cell focus-forming and HIX-MLV analyzed previously. The map locations of the 23 SFFV T1-oligonucleotides relative to the 3'' polyadenylic acid coordinate of SFFV RNA were deduced from the size of the smallest polyadenylic acid-tagged RNA fragment from which a given oligonucleotide was isolated. The resulting oligonucleotide map could be divided roughly into 3 segments: 2 terminal segments which were mosaics of oligonucleotides of classes i, ii, and iii and an internal segment between 2-2.5 kilobases from the 3'' end containing the two oligonucleotides shared with amphotropic MLV. Since SFFV RNA consisted predominantly of sequence elements related to ecotropic and amphotropic helper-independent MLV, the transforming gene of SFFV is apparently not a major specific sequence unrelated to genes of helper viruses, as is the case with Rous sarcoma and probably with other defective sarcoma and acute leukemia viruses.