Distribution of ribosomal genes in nucleoli of herpes simplex virus type 1 infected cells

Abstract

Herpes simplex virus type 1 (HSV-1) infection of HeLa cells induces profound changes in the structure of the nucleoli. They become markedly elongated, and their fibrillar centers become greatly diminished in number, but larger than in non-infected HeLa cells, and only partially surrounded by the dense fibrillar component. The effect of prolonged HSV-1 infection on the distribution of the rRNA genes was studied by means of postembedding electron microscope in situ hybridization using a biotinylated ribosomal DNA (rDNA) probe, which spans about half of the rRNA gene, and subsequent immunogold labeling of the resulting hybrids. Gold particles accumulated over two structures: a large solitary, finely fibrillar, moderately electron opaque area which was detectable only in a few sections of nucleoli and corresponded to the virus-modified fibrillar center, and over limited areas of the nucleolus-associated chromatin. In non-infected HeLa cells, foci of clustered rRNA genes were observed in the more frequently detected fibrillar centers and in association with condensed chromatin. It would be expected that foci of extended rDNA molecules might contain active or potentially active genes, whereas foci of highly compacted rDNA molecules might contain inactive genes. The ribosomal RNA molecules which were detected with the same probe over the dense fibrillar component and the granular component of the nucleoli of both infected and non-infected cells were not found within the rDNA-containing foci. The data strongly suggest that the changes in the size and number of fibrillar centers induced by the intranuclear development of HSV-1 might be directly linked to the well-known decrease of the nucleolar activity.