INTRON-DEPENDENT RECRUITMENT OF PRE-MESSENGER-RNA SPLICING FACTORS TOSITES OF TRANSCRIPTION

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