U1 and U2 small nuclear RNAs are present in nuclear speckles.

The localization of U1 and U2 small nuclear RNAs (snRNAs) has been examined by in situ hybridization using 2'-O-alkyl oligonucleotide probes. We have found that these snRNAs, which are essential for pre-mRNA splicing, localize in a speckled distribution, in addition to being present in three of four foci, in HeLa cell nuclei. However, in cells of defined passage, such as Detroit 551 and WI-38 fibroblasts, these snRNAs are concentrated in nuclear speckles, and foci are not observed. The speckled distribution of U1 and U2 snRNAs is coincident with the speckled regions enriched in small nuclear ribonucleoprotein particle (snRNP) proteins and the essential non-snRNP splicing factor SC-35. The localization of these key components of the pre-mRNA splicing machinery to speckled nuclear regions suggests that these regions may be involved in pre-mRNA splicing.