Intranuclear distribution of poly(A) RNA determined by electron microscope in situ hybridization.

We used a biotinylated poly(dT) probe to localize poly(A) RNA in HeLa cells at optical and electron microscope levels. We established that the fluorescent speckled staining pattern corresponds at the ultrastructural level to the labeling of perichromatin fibrils, at least part of the population of perichromatin granules, and clusters of interchromatin granules. Coiled bodies and the interchromatin granule-associated zones, a recently described subcompartment containing U1 but not U2 snRNA, were not labeled. The density of the labeling of interchromatin granule clusters exceeded by three to five times that of the surrounding extranucleolar area. These results are discussed in relation to the role of perichromatin fibrils in splicing of pre-mRNA and to the possible involvement of interchromatin granules in the assembly of mature spliceosomes as well as in sorting and/or coordination of RNA molecules to be transported to the cytoplasm.