Murine leukaemia virus RNA transcription from chromatin of normal and infected BALB/c spleen

STUDIES on Rous sarcoma virus (RSV), murine leukaemia virus (MuLV) and mammary tumour virus (MTV) systems have shown that at least parts of viral genomes are present in multiple copies in the natural host DNA. RSV-specific nucleotide sequences have been detected in various chick cells, including Rous sarcomas and embryos with or without RSV gs antigens and chick helper factor^1,2. Similarly, normal and virus-transformed mouse and rat cells contained equivalent amounts of MuLV-specific DNA sequences³. MTV-specific DNA genomes have been detected in equal numbers in mouse strains with both high and low incidences of mammary tumours⁴. In addition, induction of leukaemia viruses from apparently uninfected mouse and chicken cells indicates that complete, 'endogenous' viral genomes are present in these cells^5-8. But the integration of 'exogenous' viral genomes, upon infection with an oncogenic RNA virus, cannot be proved in an homologous system because of the lack of specific markers which could distinguish between exogenous and endogenous MuLV genes. Actual integration was demonstrated in the case of some heterologous systems, upon infection of duck and of mouse cells with RSV⁹.