DNA copies of viral RNA in rat cells transformed by Rous sarcoma virus (RSV)

Characterization of exogenous proviral sequences in hamster tumor cell lines transformed by Rous sarcoma virus rescued from XC cells

Alterations in viral structural genes have been studied in five cell lines derived from Syrian hamster tumors which had been induced by the virus rescued from XC cells by transfection. Two cell lines, H-18 and H-20, have all the viral structural genes expressed, but a new EcoRI recognition site appeared in the region of the pol gene sequence. Provirus present in H-12 lacks the 3' part of the gag gene sequences as well as the pol gene, therefore, it gives rise to an anomalous 1.8 Md EcoRI fragment. This line also does not synthesize viral RNA of genomic size, and none of the subgenomic RNAs found hybridized with the DNApol probe. The H-19 cell line harbors only the src gene and LTR sequences, the U3 part of which seems incomplete or different from that of PR-RSV. The cryptic proviral structure in H-19 is transcribed into src mRNA. The degree of transcription of the src gene is about 25 viral RNA equivalents per cell. The H-9 cells harbor the complete provirus and, in addition, proviral structures having the deletion in gag-pol genes. The possible ways of development of provirus alterations and the role of cryptic proviral sequences in oncogenesis are discussed.

Helper virus-dependent RSV rescue from two lines of hamster cells transformed in vivo by XC RSV

Newborn hamsters were inoculated with chicken sarcoma tissue produced by XC RSV which had been rescued by transfection from XC cells. Tumours with a latency of up to 20 months appeared in all inoculated hamsters. Three tumour cell lines were successfully established for in vitro growth and cloned. The virus was rescued from the PR RSH-9 cell line by cell fusion with chicken fibroblasts and from the other two lines (PR RSH-12 and PR RSH-14) and clones only if chicken fibroblasts used for fusion had been preinfected with a helper virus. The last two cell lines evidently harboured a functional env gene since the the synthesis of transforming virus started soon after they were found with 16Q cells producing defective BH-RSH(-) lacking the envelope glycoprotein. Produced viruses acquired subgroup C specificity of XC rsv originally used for transformation of these two cell lines. The nature of provirus alterations in PR RSH-12 and PR RSH-14 cells as well as the mechanisms underlying these changes are discussed.

Presence in leukemic cells of avian myeloblastosis virus-specific DNA sequences absent in normal chicken cells

(3)H-labeled 35S RNA from purified avian myeloblastosis virus (AMV) was exhaustively hybridized with an excess of normal chicken DNA to remove all viral RNA sequences which are complementary to DNA from uninfected cells. The [(3)H]RNA which failed to hybridize was isolated by hydroxylapatite column chromatography which separates DNA-RNA hybrids from single-stranded [(3)H]RNA. The residual RNA hybridized to leukemic chicken DNA but did not rehybridize with normal chicken DNA. This demonstrates conclusively that DNA from AMV-induced leukemic cells contain viral-specific sequences which are absent in DNA from normal cells.

Acquisition of New DNA Sequences After Infection of Chicken Cells with Avian Myeloblastosis Virus

DNA-RNA hybridization studies between 70 S RNA from avian myeloblastosis virus (AMV) and an excess of DNA from (i) AMV-induced leukemic chicken myeloblasts or (ii) a mixture of normal and of congenitally infected K-137 chicken embryos producing avian leukosis viruses revealed the presence of fast- and slow-hybridizing virus-specific DNA sequences. However, the leukemic cells contained twice the level of AMV-specific DNA sequences observed in normal chicken embryonic cells. The fast-reacting sequences were two to three times more numerous in leukemic DNA than in DNA from the mixed embryos. The slow-reacting sequences had a reiteration frequency of approximately 9 and 6, in the two respective systems. Both the fast- and the slow-reacting DNA sequences in leukemic cells exhibited a higher T m (2 C) than the respective DNA sequences in normal cells. In normal and leukemic cells the slow hybrid sequences appeared to have a T m which was 2 C higher than that of the fast hybrid sequences. Individual non-virus-producing chicken embryos, either group-specific antigen positive or negative, contained 40 to 100 copies of the fast sequences and 2 to 6 copies of the slowly hybridizing sequences per cell genome. Normal rat cells did not contain DNA that hybridized with AMV RNA, whereas non-virus-producing rat cells transformed by B-77 avian sarcoma virus contained only the slowly reacting sequences. The results demonstrate that leukemic cells transformed by AMV contain new AMV-specific DNA sequences which were not present before infection.