Inability of the nondefective Rous sarcoma provirus to generate, upon transfection, a transformation-defective virus

Tandem Integration of Avian Retroviruses in Double-Infected Cells

“…Hanafusa. Nondefective (nd) transforming viruses were assayed by focus formation in the presence of 2 ¡xg/ml Polybrene (ALDRICH-EUROPE, Bccrse, Belgium) to en hance virus adsorption in assays with viruses of any subgroup other than A. Transformation-defective(td) viruses were assayed by endpoint dilution; the td virus in infected cultures was detected by reverse tran scriptase assay [17].…”

“…After the DNA treat ment, the cells were subcultured 3 times during 3 weeks and examined both for foci of transformed cells and by reverse transcriptase assay [17] for nontransforming virus. The specific infectivity, in terms of infectious units per micrograni DNA, was determined at the end point dilution from the fraction of the virus-positive cultures.…”

Transformation-Defective Virus Generation from Cloned Nondefective Avian Sarcoma Virus

“…There may be difficulty in picking a focus of ASV-transformed cells free of nontransformed cells which may be con taminated with td viruses. Hillova et al [5] found after cloning ASV-infected cells in soft agar that infectious DNA from such cloned cells gave rise to nd ASV only with no td viruses. In an attempt to isolate an ASV free of td viruses and to investigate the mechanism of generation of td viruses, the following study was carried out with an ASV which was cloned twice in soft agar, without intervening passage of the virus.…”

In vitro Synthesis of Infectious DNA Copies of the Rous Sarcoma Virus Genome

“…Chicken cells used in these assays were obtained from Brown Leg horn embryos as described in the text. Occurrence of transforming and transformation-defective (td) viruses in transfected cultures was detected by focus-formation and reverse transcriptase [20] assay, respectively. The amount of infectious units (IU) in the reaction product was determined, by using Poisson distribution, from the fraction of positive cultures at the endpoint dilu tion, as described [21], In the last experiment, 3 x 108 FFU were incubated in 2 ml of the reaction mixture without the four dNTPs.…”