Splicing of nuclear pre-mRNA is an important step in the regulation of gene expression as only correctly spliced mRNAs will be exported to the cytoplasm to function in protein synthesis, Nuclear RNA transcripts of split genes and their splicing products, as well as the general population of nuclear polyadenylated RNA, ape packaged in multicomponent large nuclear ribonucleoprotein (lnRNP) particles. These InRNP particles, which sediment at the 200S region in sucrose gradients, contain all U snRNPs required for pre-mRNA splicing and several protein splicing factors, including U2AF and the SR proteins and can thus be viewed as naturally assembled complexes of pre-mRNA and splicing factors, We have previously reconstructed. the three-dimensional image of negatively stained individual lnRNP particles by automated electron tomography. The reconstruction revealed a compact structure, 50 nm in diameter, composed of four major subunits, Here we further analyzed the reconstructed models and the apparent connectivity between the subunits using anew rendering technique, The uniformity of the lnRNP particles was substantiated by measurement of the volume engulfed by their surface, This study further supports the model proposed for the packaging of nuclear pre-mRNAs in lnRNP particles, where each substructure represents a functional unit. This model is compatible with the requirements for alternative splicing in multiintronic pre-mRNAs, and with the fact that the splicing of multiintronic pre-mRNAs does not occur in a sequential manner. (C) 1997 Academic Press.