Organization of RNA polymerase II transcription and pre-mRNA splicing within the mammalian cell nucleus.

Introduction Splicing is an essential step in the processing of intron-containing pre-mRNAs before their transport out of the nucleus (for a review see [ 13). During the last few years, a significant number of studies have addressed the relationship of pre-mRNA splicing to nuclear structure (for a review see [Z]). Small nuclear ribonucleoprotein particles (snRNPs) as well as several non-snRNP splicing factors have been shown by immunofluorescence microscopy to be localized in a speckled pattern that is distributed in various regions throughout the nucleoplasm. At the electron microscopic level, the speckled pattern corresponds to interchromatin granule clusters and perichromatin fibrils [ 3-61. After pulse-labelling with [ 'Hluridine, incorporation is observed preferentially by electron microscopy at perichromatin fibrils, with little to no labelling of internal regions of interchromatin granule clusters (for a review see [7] ) . Based on these autoradiography and immunolocalization studies, it has been suggested that perichromatin fibrils may represent the sites of active transcription and interchromatin granule clusters, sites of splicing factor storage and/or assembly. W e have examined the relationship of the organization of pre-mRNA splicing factors to the transcriptional and splicing activities of the cell. We have found that splicing factors shuttle between storage and/or assembly sites (interchromatin granule clusters) and sites of active transcription (perichromatin fibrils). Our data suggest a nuclear recruiting mechanism that regulates the distribution of splicing factors.