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If pre-mRNA splicing begins during RNA synthesis, then transcriptionally active genes may be expected to contain high concentrations of pre-mRNA splicing factors. However, previous studies have localized splicing factors to a network of "speckles," which is distinct from individual sites of gene transcription where pre-mRNA is spliced. Speckles have been detected with antibodies specific for splicing snRNPs and members of the SR family of splicing factors. Here we report that dilution of these probes results in the visualization of hundreds of sites throughout the HeLa cell nucleus, the size and distribution of which are consistent with transcription units viewed with light microscopy. Importantly, these sites of highest SR protein concentration frequently coincide in three-dimensional space with active sites of RNA polymerase II transcription. A newly developed reagent specific for a single member of the SR family, SRp20, detects a subset (approximately 20%) of these sites, suggesting the gene-specific accumulation of these splicing regulators, which have distinct functions in pre-mRNA splicing. These observations question the view that the nucleus and its functions are highly compartmentalized; instead, they support a model in which the localization of these and possibly other gene regulators is determined primarily by their function.

Distribution of pre-mRNA splicing factors at sites of RNA polymerase II transcription.