INTRANUCLEAR DISTRIBUTION OF U1 AND U2 SNRNA VISUALIZED BY HIGH-RESOLUTION IN SITU HYBRIDIZATION - REVELATION OF A NOVEL COMPARTMENT CONTAINING U1 SNRNA BUT NOT U2 SNRNA IN HELA-CELLS

We have examined the intranuclear distribution of U1 and U2 small nucl ear RNAs (snRNAs) in HeLa cells by electron microscope in situ hybridi zation using biotinylated DNA probes reacting at the surface of thin s ections of Lowicryl-embedded cells. U1 and U2 snRNAs colocalized on pe richromatin fibrils, clusters of interchromatin granules and coiled bo dies. The perichromatin granules were just occasionally labeled. In ad dition, we identified a novel nuclear domain associated with the clust ers of interchromatin granules which contains U1 but not U2 snRNA. Thi s new compartment termed ''interchromatin granule-associated zone'' ha s a fibrillar texture, does not contain DNA and might be the equivalen t of the A snurposomes described in germinal vesicles of amphibians (W u et al., J. Cell Biol. 113, 465-483 (1991)). We propose that the inte rchromatin granule-associated zones might be sites of the final matura tion of the U1-pre-snRNP particle before its transfer to interchromati n granules and its subsequent assembly in the spliceosome.