NUCLEAR-ORGANIZATION AND GENE-EXPRESSION

In actively transcribing cells, factors involved in pre-mRNA splicing localize in a speckled pattern at the fluorescence microscopic level. The speckled pattern corresponds to interchromatin granule clusters an d perichromatin fibrils at the electron microscopic level. Based upon [H-3]uridine incorporation studies transcription is thought to occur a t the perichromatin fibrils and not within interchromatin granule clus ters. We have shown that upon inhibition of RNA polymerase II transcri ption or pre-mRNA splicing, splicing factors redistribute and preferen tially localize to interchromatin granule clusters, which become large r and more uniform in shape. Introduction of exogenous DNA templates i nto the cell nucleus results in a recruitment of splicing factors to t he new sites of transcription. These data suggest that splicing factor s are localized at storage and/or reassembly sites (interchromatin gra nule clusters) and are recruited to active sites of transcription (per ichromatin fibrils). Furthermore, these data demonstrate that the spec kled localization of pre-mRNA splicing factors is a reflection of the transcriptional and pre-mRNA splicing activities Of the cell. (C) 1996 Academic Press, Inc.