We have examined the nuclear localization of transiently and stably expressed nascent RNA transcripts containing or lacking introns in order to determine if the spatial association of RNA transcripts and pre-mRNA splicing factors in nuclei is random or functionally significant. Our findings show that the association between nascent RNA and splicing factors in the nucleus is intron-dependent when the RNAs are either transiently or stably expressed. Furthermore, our data indicate that splicing factors are recruited to the transcription sites. The presence of both pre-and mRNA at these locations suggest that pre-mRNA splicing occurs at these sites of transcription. In addition, electron microscopic examination of the highly active transcription sites has revealed a granular appearance which closely resembles, but is functionally different from, interchromatin granule clusters. Our findings demonstrate that the nucleus is highly organized and dynamic with regard to the functions of the transcription and pre-mRNA splicing.

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