Ingela Wetterberg ^{1, 2}, Jian Zhao ², Sergej Masich ¹, Lars Wieslander ^{2,  3} and Ulf Skoglund ¹

¹ Department of Cell and Molecular Biology, Medical Nobel Institute, Karolinska Institute, SE-171 77 Stockholm, Sweden;
² Department of Molecular Biology and Functional Genomics, Stockholm University, SE-106 91 Stockholm, Sweden;
³ Corresponding author e-mail: Lars.Wieslander@molbio.su.se 

The Balbiani ring 3 (BR3) gene contains 38 introns, and more than half of them are co-transcriptionally excised. We have determined the in situ structure of the active BR3 gene by electron tomography. Each of the 20–25 nascent transcripts on the gene is present together with splicing factors and the RNA polymerase II in a nascent transcript and splicing complex, here called the NTS complex. The results indicate that extensive changes in overall shape, substructure and molecular mass take place repeatedly within an NTS complex as it moves along the gene. The volume and calculated mass of the NTS complexes show that, maximally, one complete spliceosome is assembled on the multi-intron transcript at any given time point. The structural data show that the spliceosome is not a structurally well-defined unit in situ and that the C-terminal domain of the elongating RNA polymerase II cannot carry spliceosomal components for all introns in the BR3 transcript. Our data indicate that spliceosomal factors are continuously added to and released from the NTS complexes during transcription elongation. 

1. "Ultrastructural Continuity Between Active and Repressed Chromatin".

2. "Oncogenes as Molecular Targets within Active Chromatin".