Intranuclear distribution of U1 and U2 snRNAs visualized by high resolution in situ hybridization: revelation of a novel compartment containing U1 but not U2 snRNA in HeLa cells.

We have examined the intranuclear distribution of U1 and U2 small nuclear RNAs (snRNAs) in HeLa cells by electron microscope in situ hybridization using biotinylated DNA probes reacting at the surface of thin sections of Lowicryl-embedded cells. U1 and U2 snRNAs colocalized on perichromatin fibrils, clusters of interchromatin granules and coiled bodies. The perichromatin granules were just occasionally labeled. In addition, we identified a novel nuclear domain associated with the clusters of interchromatin granules which contains U1 but not U2 snRNA. This new compartment termed "interchromatin granule-associated zone" has a fibrillar texture, does not contain DNA and might be the equivalent of the A snurposomes described in germinal vesicles of amphibians (Wu et al., J. Cell Biol. 113, 465-483 (1991)). We propose that the interchromatin granule-associated zones might be sites of the final maturation of the U1-pre-snRNP particle before its transfer to interchromatin granules and its subsequent assembly in the spliceosome.