DYNAMIC RELOCATION OF TRANSCRIPTION AND SPLICING FACTORS DEPENDENT UPON TRANSCRIPTIONAL ACTIVITY

Recent interest in understanding the spatial organization of gene expr ession has focused attention on nuclear structures known as speckles o r interchromatin granule clusters (IGCs) revealed by immunofluorescenc e or electron microscopy, Staining of nuclear factors involved in pre- mRNA splicing or, more recently, transcription, reveals 20-40 speckles per nucleus, resulting in the intriguing suggestion that speckles are nuclear sites of transcription and processing, In contrast, other inv estigations have observed transcription in other areas of the nucleus, In this study, we have examined the localization of active transcript ion as detected by uridine incorporation and recently developed RNA po lymerase II antibodies, and compared this pattern with that of known s plicing and polyadenylation factors, Our results indicate that in acti vely transcribing cells, transcription and splicing factors are disper sed throughout the nucleus with abundant sites of preferred localizati on, In contrast, in poorly transcribing cells, polymerase II and splic ing factors localize to speckles, In nuclei inactivated for transcript ion by drugs or heat shock, the speckle type of co-localization is acc entuated, These observations suggest that bulk transcription and splic ing occur throughout the nucleus during periods of active transcriptio n; and that factors involved in these two processes re-locate to minim al speckle domains during periods of inactive transcription.