Abstract
Virus induction in heterokaryons formed by non‐permissive cells transformed by Rous sarcoma virus (RSV) and permissive chick embryo (CE) cells, in the presence of inactivated Sendai virus, was studied with a clone of hamster BHK21 fibroblasts transformed by Schmidt‐Ruppin strain RSV (clone RS2) and subclones derived from this clone. The main results have been the following: (1) The number of heterokaryons giving rise to foci of Rous cells (focus‐forming infective centers, or FFICs) in mixed cultures is proportional to the number of RS2 cells when the initial CE:RS2 cell ratio attains 30, and almost all heterokaryons are then formed from single RS2 cells. (2) Recovery of FFICs also depends on the dose of Sendai virus. (3) Following fusion of RS2 cells with cells from different chick embryos the recovery of FFICs varies in parallel with the number of foci produced by free virus in cultures of these embryos. (4) The latent period of virus induction from the time of cell fusion is about 24 h, and somewhat longer than that of viral replication in newly infected CE cells. The growth of induced virus is similar to that of virus produced after de novo infection. (5) Virus induction requires no DNA synthesis but is inhibited, reversibly, by treatment for a few hours with inhibitors of DNA‐dependent synthesis (actinomycin D) and protein synthesis (puromycin and cycloheximide). Hence, non‐permissiveness of RS2 cells may be due to a partial block of transcription of the viral replicative DNA (provirus) present in the cells, and induction in heterokaryons to switching from partial to full transcription and translation of the provirus. However, it is also possible that some function, or factor (s) of the CE cells is required for virus production in heterokaryons. (6) The study of a subclone of RS2 cells 100‐ to 1000‐fold less inducible than other subclones suggesting that, in this subclone, non‐permissiveness may be due to a permanent block of synthesis of all viral proteins, at the level of transcription of the provirus.