We have studied SV40 (simian virus 40) nucleoprotein in permissively infected monkey kidney cell cultures (CV1) by a procedure which does not require the isolation of the SV40 chromosomes. Treatment of the cells by a low ionic strength medium containing Photo flo produces a mild loosening of nucleoproteins, and permits the study in ultrathin sections of virus components and their relationships with host cell chromatin. RNP and DNP could be distinguished by uranyl-EDTA-lead staining (for RNP) and by DNase digestion. SV40 DNA was observed as circular molecules, either free or connected with either RNP fibrils or virus capsids. These three aspects were interpreted, respectively, as viral minichromosomes, transcription of virus genome and partially encapsidated virus DNA. During encapsidation a few virus particles appear to be bound to host chromatin. Many, if not all, seemingly mature viruses, singly or in small linear clusters, are also aligned on host chromatin. Some of these observations were corroborated by the Miller spreading technique. They are consistent with a role for the host cell chromatin in the production of nuclear viruses.


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