Along with other carcinogens of physical or chemical origin, viruses are known to be associated with and to cause tumors in a variety of experimental animals. RNA tumor viruses in particular are widespread in many species of animals, and frequently cause sarcomas or leukemia. The isolation of fowl sarcoma-leukemia virus in the early 1910s by a number of investigators marked the first successful identification of such tumor viruses (Ellermann and Bang, 1909; Rous, 1911; Fujinami and Inamoto, 1914). The cell-virus systems used for the studies of basic aspects of the pathogenesis of avian tumor viruses illustrate the progress in methodology for studying animal viruses in general. Early work in animal hosts was gradually replaced by a system using the chorioallantoic membrane of eggs (Keogh, 1938), then by tissue culture cells (Manaker and Groupe, 1956). The establishment of an assay system for Rous sarcoma virus (RSV) in tissue culture cells (Temin and Rubin, 1958) eventually led to an era of quantitative studies on the mechanism of cellular alteration by viruses.

Cell Transformation by RNA Tumor Viruses

Viral complementary DNA (cDNA) sequences corresponding to the gag, pol, env, src, and c regions of the Rous sarcoma virus genome were selected by hybridizing viral cDNA to RNA from viruses that lack the env or src gene or to polyadenylic acid [poly(A)]-containing RNA fragments of different lengths and isolating either hybridized or unhybridized DNA. The specificities, genetic complexities, and map locations of the selected cDNA's were shown to be in good agreement with the size and map locations of the corresponding viral genes. Analyses of virus-specific RNA, using the specific cDNA's as molecular probes, demonstrated that oncovirus-infected cells contained genome-length (30-40S) RNA plus either one or two species of subgenome-length viral RNA. The size and genetic content of these RNAs varied, depending on the genetic makeup of the infecting virus, but in each case the smaller RNAs contained only sequences located near the 3' end of the viral genome. Three RNA species were detected in Schmidt-Ruppin Rous sarcoma virus-infected cells: 39S (genome-length) RNA; 28S RNA, with an apparent sequence of env-src-c-poly(A); and 21S RNA, with an apparent sequence of src-c-poly(A). Cells infected with the Bryan high-titer strain of Rous sarcoma virus, which lacks the env gene, contained genome-length (35S) RNA and 21S src-specific RNA, but not the 28S RNA species. Leukosis virus-infected cells contained two detectable RNA species: 35S (genome-length) RNA and 21S RNA, with apparent sequence env-c-poly(A). Since gag and pol sequences were detected only in genome-length RNAs, it seems likely that the full-length transcripts function as mRNA for these two genes. The 28S and 21S RNAs could be the active messengers for the env and src genes. Analyses of sequence homologies among nucleic acids of different avian oncoviruses demonstrated substantial similarities within most of the genetic regions of these viruses. However, the "common" region of Rous-associated virus-0, an endogenous virus, was found to differ significantly from that of the other viruses tested.

Size and genetic content of viral RNAs in avian oncovirus-infected cells.

The genetic analysis of RNA tumor viruses has two main objectives: (1) to provide an understanding of virus replication and (2) to explain virus-induced transformation of the host cell. Virus replication results from a complex interaction of viral and cellular genomes. Viral genetics, however, considers only the virus side of this interaction; the numerous and specific cellular functions which are required for the synthesis of infectious virus will have to be defined by a genetic analysis of the host cell. In virus-induced transformation, viral genetic information presumably interferes with the genetic regulatory apparatus of the host cell, and here again it is important to realize that focusing on the viral information will reveal only part of a very complex interaction between two organisms. Despite these obvious limitations, the viral genome is at the moment that partner in this interaction which is more amenable to experimental study and offers realistic opportunities for an increase of our insight into virus replication and cellular transformation.

Anthony L. Schincariol

Papers

Early synthesis of virus-specific RNA and DNA in cells rapidly transformed with Rous sarcoma virus