Intranuclear retention of ribosomal RNAs in response to herpes simplex virus type 1 infection

The localization of ribosomal RNA (rRNA) was investigated at the ultrastructural level in herpes simplex virus type 1 infected HeLa cells using three distinct biotinylated probes which bind in sequence to three different segments of the ribosomal genes. Comparison of the above with the signal levels obtained from non-infected cells reveals information about the effects of HSV-1 infection on ribosome biogenesis. A probe specific for the 5' end portion of pre-rRNA labeled all nucleoli of both non-infected and infected cells in the same way, that is, it mainly labeled the dense fibrillar component and the border of the fibrillar centers but only slightly labeled the granular component. This indicates that the initial cleavage of pre-rRNA in herpes infection still occurs in the 5' region of the 5' external transcribed spacer. However, a probe specific for 18 S rRNA labeled the granular component of the nucleoli more intensely after infection. In addition, significant amounts of rRNA molecules were present within the intranuclear viral region, except over the enclosed viral dense bodies, and within the virus-enlarged clusters of interchromatin granules. The data indicate that the still enigmatic viral dense bodies, which are nucleolus-related structures, are excluded from the marked intranuclear retention of ribosomal RNAs and, in addition, reveal a possible role for the interchromatin granules of infected cells in the regulation of the export of the ribosomal subunits towards the cytoplasm.