Dynamics of transcription and pre-mRNA splicing within the mammalian cell nucleus

Splicing is an essential step in the processing of intron-containing pre-mRNAs prior to their transport out of the nucleus (for review, see Green 1991). During the last few years, a significant number of studies have addressed the relationship of pre-mRNA splicing to nuclear structure (for review, see Spector 1993). 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. Poly(A)+ RNA has been shown to colocalize with these nuclear speckles (Carter et al. 1991, 1993). Several specific RNAs have been localized in association with, the periphery of one or more speckles in a particular cell nucleus (Huang and Spector 1991; Xing et al. 1993). At the electron microscopic level, the speckled pattern corresponds to interchromatin granule clusters and perichromatin fibrils (Perraud et al. 1979; Fakan et al. 1984; Puvion et..