TUMOR VIRUS RNAs AND TUMOR VIRUS GENES

The RNAs of several classes of avian and murine RNA tumor viruses were compared. The 60-70s RNA complex of cloned nondefective avian sarcomaviruses contains only 30-40s RNA species of class a, which are larger than the 30-40s RNA species of class b found in avian transformation-defective or leukosis viruses. The RNA of a recombinant avian sarcoma virus, carrying a host range marker of a leukosis virus parent, also consists only of class a subunits. This implies that recombination arnong avian tumor viruses involves crossing-over rather than reassortment. The class a RNA subunits of nondefective avian sarcoma viruses and the class b RNAs of transformation-defective, leukosis viruses of the Same subgroup and strain have very similar oligonucleotide-fingerprint Patterns and are at least 60 % homologous if compared by RNA-DNA hybridization. It is suggested that the class b RNA of leukosis viruses is a deletion of class a RNA from corres~onding nondefective sarcoma viruses and that their structural relationship may be expressed as a = b + X. We assume, that X represents genetic information directly or indirectly involved in transformation of fibroblasts. Passage at high multiplicity of cloned sarcoma viruses, containing only 30-40s RNA of class a, led to the appearance of 30-40s RNA of class b in progeny virus. Two replication-defective avian sarcoma viruses (Bryan RSV and MC 29) lack 30-40s RNA of class a. They probabl~ contain distinct types af 30-40s RNA resembling class b RNA of leukosis viruses in size but differing in composition. The Kirsten murine sarcoma virus, which appears to be more defectice than Bryan RSV or MC 29, has a 30-40s RNA which is even srnaller than of its leukosis helper virus. These observations suggest that a correlation exists between the size of the viral RNA species and defects in transformation-and/or replication genes of the corresponding viruses. The greater the extent of the defectiveness, the smaller is the size of the viral RNA. Possible mechanisms generating deletions in tumor virus RNA are discussed. Different classes of RNA tumor viruses: Two classes of RNA tumor viruses can be readily distinguished on the basis of their pathological effects on cells (1-6): (i) Sarcoma viruses, which transform fibroblasts in tissue culture and cause solid tumors in the animal and (ii) leukosis or leukemia viruses, most of which fail to transform fibroblasts in tissue culture, but ciuse a lymphatic or myeloblastic leukemia in the animal. A possible exception appear to be some endogenous leukosis-type viruses; for example, the endogenous avian leukosis virus RAV (0) has so far not been found to cause a disease in chicken (Vogt, P. K., G. Purchase and R. Weiss (1973) personal communication). Endogenous murine leukemia virus of AKR-mice has the Same serotype and N-tropism as known leukemogenic virus strains, however it has not yet been tested whether it causes leukemia in mice (Rowe, W. P. (1973) personal communcation). Since in the avian tumor virus group, sarcoma and leukosis viruses have been found which resemble each other in most biological and biochemical properties except transforming ability for fibroblasts (4, S), many, and ~ e r h a ~ s all avian leukosis viruses may be considered as defective sarcoma viruses which have lost their ability to transform fibroblasts. Consequently those avian leukosis viruses which have been derived in the laborator~ (4, 5) from sarcoma viruses have been termed transformation-defective (td) viruses (7). In addition replication-defective sarcoma viruses exist in the avian (8) and murine tumor virus group (9). These may be considered sarcoma viruses which have lost all or Part of the genetic information required for virus-replication and/or virus-structure. All replication-defective RNA tumor viruses rely of necessity on associated helper viruses or helper cells to complement their defective structural or replicative functions (8, 10). Since the known helper viruses do not repair, by genetic recombination, the defectiveness of replication-defective sarcoma viruses, their defectiveness is genetically stable (8, 10). If we were to set up a hierarchy of decreasing genetic potentials among different classes of RNA tumor viruses we could distinguish the following categories: Firstly, the nondefective sarcoma viruses, which carry all genes essential for replication and cell transformation (examples: Prague Rous sarcoma virus (PR RSV), Schmidt Ruppin (SR) RSV, B77 RSV). From these the category of transformation-defective (td) and leukosis viruses could be formally derived by deletion of genes required for cell transformation (examples: td PR RSV, Rous associated virus (RAV) and murine leukemia virus. Similarly the replication-defective sarcoma viruses may be considered as a viral category which has lost all or Part of the genetic potential of nondefective sarcoma viruses required for virus replication (examples: Bryan RSV, Kirsten murine sarcoma virus (MSV) , Moloney MSV). In addition sarcoma viruses may exist which are both replicationand transformation-defective and leukosis viruses may exist which are replication-defective (see Fig. 8). It is the purpose of this report t o summarize and extend correlations made (3, 5, 6, 7, 26, 31) between different genetic potentials of RNA tumor viruses and various physically defined classes of viral RNA. The 60-70s RNA complex of RNA tumor viruses: RNA tumor viruses contain a 60-70s RNA corresponding to an approximate molecular weight of 1 X 10' (11). This RNA ~ r o b a b l ~ represents part or all of the viral genome (11, 12). The 60-70s RNA can be dissociated by heat or organic solvents (formamide, dimethysulfoxide) to yield molecules which sediment at 30-40s and some smaller RNA species (11, 13) and was therefore proposed to have a subunit structure (1 I). Some recent observations have lent biological credibility to the possible subunit structure of viral 60-70s RNA. It was found that avian sarcoma viruses harvested a 3 min intervals from cells contain 30-40s instead of 60-70s RNA. Upon incubation of such virus at 40° the 30-40s RNA species is converted to 60-70s RNA (14). Hence the 30-40s subunits appear to be precursors rather than fragments of 60-70s RNA (14). The 30-40s RNA species of nondefective avian sarcoma viruses has been resolved electrophoretically into two size classes, a and b (3). Electrophoresis in formamide gels indicates that class b RNA is about 12 % smaller than class a RNA (13). Molecular weight estirnates of class a RNA in formamide gels are between 2.9 and 3.4 x 106 depending on the reference RNA used as marker (13). The presence of class a RNA is well correlated with viral ability to induce focus formation in fibroblast cultures (7). Exceptions to this rule are replication-defective viruses such as the Bryan RSV and avian leukosis Virus strain MC 29 which have focus forming ability but contain RNA which rnigrates with size class b (5, 6). Avian leukosis viruses and transformationdefective derivatives of sarcoma viruses contain only class b RNA (3, 5, 6, 7). I Ii (-1 PR RSV-C Clone 4 I I I I I a B I (U) PR RSV-C Clone 12