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The structure of the genomic RNAs and the gene products of avian Fujinami sarcoma virus (FSV) and of avian acute leukemia viruses with sarcomagenic potential, i.e., viruses of the myelocytomatosis (MC29)- and erythroblastosis (AEV)-subgroups, are compared. The genome of replication-defective FSV is a 4.5 kilobase (kb) RNA and, like the RNAs of defective acute leukemia viruses, consists of three distinct sections: an internal FSV-specific section flanked by 5′ and 3′ helper-virus-related sequences. Their related terminal sequences identify each of these viruses as belonging to the avian tumor virus group. The internal specific sequences are the structural basis for the distinction of the different subgroups of avian oncogenic viruses, i.e., the MC29-, AEV-, and FSV-subgroups. These sequences are not shared between members of different subgroups, and are not related to the src gene of the Rous subgroup of sarcoma viruses. In each of these viruses, the 5′- gag gene-related and internal specific sequences together form a genetic unit that codes for a nonstructural phosphoprotein which is probably involved in transformation. These genetic units represent less than the total coding capacity of the acute leukemia viral RNAs. However, in the case of FSV, the genetic complexities of the 4.5-kb RNA and of the 140,000-dalton, FSV-specific protein are nearly identical, a fact which excludes the synthesis of other major proteins, in the same reading frame, and suggests that this protein would be sufficient for transforming function. The 140,000-dalton FSV protein is phosphorylated in an in vitro kinase reaction by itself or an associated phosphokinase.

The map location of the gene for viral DNA polymerase on Rous sarcoma virus RNA was investigated. For this purpose specific oligonucleotides of a sarcoma virus carrying a temperature-sensitive (ts) polymerase were sought which would segregate with all viral recombinants containing a ts polymerase marker. To detect such oligonucleotides, the chemical map locations of 20 to 30 RNase T1-resistant oligonucleotides within the 10,000 nucleotides of the RNA were determined and oligonucleotide maps were constructed. The location of each oligonucleotide within the map was derived from the length of the smallest poly (A) tagged-RNA fragment from which it could be isolated. By comparing the oligonucleotide maps of (i) a sarcoma virus with a ts polymerase PR-C LA337, termed ts337, (ii) a leukosis virus, RAV-6 of subgroup B and (iii) of 15 recombinants selected for a ts or wild type polymerase and two other parental markers e.g. the envelope gene of ts337 or RAV-6, termed env , and the sarcoma gene of ts337, termed src we found the following: (i) One map segment between 6000 and 8000 nucleotides from the poly (A) end, defined by four oligonucleotides, was shared by all parental and recombinant RNAs. (ii) All other map segments in parental and recombinant RNAs contained either ts337 or RAV-6-specific oligonucleotides together with some interspersed oligonucleotides shared by both viruses.

Summary Indirect evidence suggests that the replication of RNA tumor viruses differs from that of cytocidal RNA viruses in two distinctive properties: a requirement for early DNA synthesis and one for permanent DNA dependent RNA synthesis. These requirements led to the provirus hypothesis of Temin which states that there is a DNA intermediate in RNA tumor virus replication. The provirus hypothesis has been strengthened by the discovery of virus-associated RNA dependent DNA polymerase. Yet, neither provirus hypothesis nor the virus-associated RNA dependent DNA polymerase explain the peculiar requirement of RNA tumor virus replication for cellular DNA synthesis. Numerous recent observations suggest that some RNA tumor virus genes, in some cases even all viral genes, preexist in normal cells. If these genes exist in phenotypically normal cells as unexpressed DNA, it is conceivable that “cellular” DNA synthesis is required in RNA tumor virus replication to derepress these genes. In this case the genetic role of the virus or of the viral RNA in RNA tumor virus infection becomes a philosophical question.