Fine Structure of Virus Chromatin in Thin Sections of SV40-infected Cells: A Cytochemical and Autoradiographical Study

Abstract

SUMMARYThe ultrastructural organization of virus chromatin was studied within nuclei prepared from CV1 cells (cultured monkey kidney cells) lytically infected with SV40 virus (simian virus 40) by a procedure which allows a mild loosening of nucleoproteins. In addition to dispersed host components, DNA-containing nucleoplasmic structures could be identified as virus chromatin. Both standard staining for structure and specific staining for DNA clearly revealed in thin sections the nucleosomal structure of well-extended virus chromatin as well as alignments of virions on host chromatin. In addition, replicating and transcribing virus chromatins were abundant as revealed by high-resolution autoradiography. Therefore, the procedure of loosening of nucleoproteins used in this report also preserves the active SV40 chromatin and allows the in situ visualization in Epon sections of transcription and replication of the virus genome.

The osmium ammine-SO2 staining method for studying the in situ configuration of viral genomes in ultrathin sections of DNA virus infected cells

“…It is this mild swelling procedure which has permitted the in situ demonstration of the nucleosomal configuration of nonencapsidated SV40 genomes, studies which once could be performed only on biochemical fractions. Combined with specific DNA staining [23], it has revealed that free viral chromatin appears as individual free, occasionally twisted circles (fig 5b, c). These circles consist of thin circular DNA fibers, 0.5 to 0.6 pm long and 3 nm thick, which bear empty spheres, 12 nm in diameter.…”

“…Unfortunately, the non-encapsidated genomes and encapsidated genomes are not easy to distinguish. At this stage of infection, host DNA is more condensed but always incompletely marginated [23].…”