Nuclear speckles (speckles) represent a distinct nuclear compartment within the interchromatin space and are enriched in splicing factors. They have been shown to serve neighboring active genes as a reservoir of these factors. In this study, we show that, in HeLa cells, the (pre)spliceosomal assembly on precursor mRNA (pre-mRNA) is associated with the speckles. For this purpose, we used microinjection of splicing competent and mutant adenovirus pre-mRNAs with differential splicing factor binding, which form different (pre)spliceosomal complexes and followed their sites of accumulation. Splicing competent pre-mRNAs are rapidly targeted into the speckles, but the targeting is temperature-dependent. The polypyrimidine tract sequence is required for targeting, but, in itself, is not sufficient. The downstream flanking sequences are particularly important for the targeting of the mutant pre-mRNAs into the speckles. In supportive experiments, the behavior of the speckles was followed after the microinjection of antisense deoxyoligoribonucleotides complementary to the specific domains of snRNAs. Under these latter conditions prespliceosomal complexes are formed on endogenous pre-mRNAs. We conclude that the (pre)spliceosomal complexes on microinjected pre-mRNA are formed inside the speckles. Their targeting into and accumulation in the speckles is a result of the cumulative loading of splicing factors to the pre-mRNA and the complexes formed give rise to the speckled pattern observed. INTRODUCTIONNuclear speckles are enriched in splicing factors and in the factors of the transcription machinery (Spector, 1990;Krause et al., 1994;Bregman et al., 1995;Larsson et al., 1995). Even though there is a consensus that these compartments play a role in RNA metabolism, their exact function is presently unknown. When growing mammalian cells are labeled with antibodies to splicing components, 20 to 50 shining nuclear domains, i.e., nuclear speckles, also termed SC35 domains (protein SC35 being an important serine/arginine (SR)-rich splicing factor [Fu and Maniatis, 1990]), splicing factor compartments, or just speckles, are usually observed (Perraud et al., 1979;Spector et al., 1983;Spector, 1990;Misteli, 2000). In some cell types, they occupy as much as 20% of the nuclear volume. At the electron microscope level, they consist of morphologically well-defined interchromatin granule clusters and of domains of perichromatin fibrils, some of which are believed to represent precursor-mRNAs (premRNAs) (Fakan and Puvion, 1980;Spector et al., 1983;Puvion et al., 1984;Spector, 1990;Fakan, 1994;Raška, 1995;Melčák et al., 2000).Most mammalian pre-mRNAs contain introns and typically have to be spliced before being transported to the cytoplasm. It has been shown biochemically that spliceosome formation and splicing may be cotranscriptional events (Wuarin and Schibler, 1994). More recent results indicate that transcription and splicing are coupled with interactions of certain factors in both processes. They take part in the large macromolecular c...