The major species of unintegrated linear viral DNA identified in chicken embryonic fibroblasts infected with either the avian myeloblastosis-associated viruses (MAV-1, MAV-2) or the standard avian myeloblastosis virus complex (AMV-S) has a mass of 5.3 x 106 daltons. An additional minor DNA component observed only in AMV-S-infected cells has a mass of 4.9 x 106 daltons. The unintegrated linear viral DNAs and integrated proviruses of MAV-1 and MAV-2 have been analyzed by digestion with the restriction endonucleases EcoRI and HindHI. MAV-2 lacks a HindIII site present in MAV-1. These fragments have been compared to those generated by EcoRI and HindIII digestion of linear viral DNAs of AMV-S. Restriction enzyme digestion of AMV-S viral DNA produced unique fragments not found with either MAV-1 or MAV-2 viral DNAs. The major viral component present in AMV-S stocks has the HindIll restriction pattern of MAV-1. Restriction enzyme analysis of the 5.3 x 106-dalton unintegrated MAV viral DNAs and their integrated proviruses suggests that the DNAs have a direct terminal redundancy of approximately 0.3 megadaltons and integrate colinearly with respect to the unintegrated linear DNA.

Avian myeloblastosis virus contains a continuous sequence of approximately 1,000 nucleotides which may represent a gene (amv) responsible for acute myeloblastic leukemia in chickens. This sequence appears to have been acquired from chicken DNA and to be substituted for the envelope gene in the viral genome. We used hybridization probes enriched for the amv sequences and conditions that facilitate annealing of partially homologous nucleotide sequences to show that cellular sequences related to amv are present in the genomes of all vertebrates ranging from amphibians to humans but were not detected in fish, sea urchins, or Escherichia coli. In contrast to the preceding findings, nontransforming endogenous proviral nucleotide sequences closely related to the remainder of the avian myeloblastosis virus genome and to the entire myeloblastosis-associated helper virus are present only in chicken DNA. The amv-related cellular sequences appear to be highly conserved during evolution and to be contained at only one or a few locations in the genome of vertebrates. Within closely related species, they appear to share common evolutionary genetic loci. These findings and similar ones obtained with other highly oncogenic retroviruses containing a transforming gene suggest a general mechanism for acquisition of viral oncogenic sequences and an essential role for these sequences in the normal cellular state.

The production and release of R-type virus-like particles (VLP) was studied in several Syrian hamster cell lines. Most but not all hamster cell lines contain detectable levels of R-type VLP. However, only BHK-21 clone F (CF) cells would release the VLP into culture fluids. Treatment with dexamethasone enhanced to a limited extent the production and release of VLP from BHK-21 CF cells. Actinomycin D inhibited the production and release of R-type VLP in hamster cells, suggesting that some transcription from DNA is necessary for VLP production. Furthermore, 5-iododeoxyuridine induced the production of R-type VLP in Syrian hamster embryo cells. These results suggest that R-type VLP are endogenous to Syrian hamsters.

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The cellular sites of integration of the avian myeloblastosis-associated virus type 2 (MAV-2) DNA have been examined by Southern blot analysis of cellular DNA from infected cloned and uncloned chicken embryonic fibroblasts. Provirus-cell juncture fragments were not detected in restriction enzyme digests of DNA from MAV-2-infected uncloned cells. However, each MAV-2-infected cell clone examined produced a unique set of junctive bands. Thse findings indicate that multiple sites of integration exists for MAV-2 proviruses in cellular DNA.