Processing of Pre-mRNA in Polytene Nuclei ofChironomus tentansSalivary Gland Cells

Salivary gland polytene cells in the dipteranChironomus tentansprovide exceptional experimental possibilities to analyze processing of specific pre-mRNAs in intact eukaryotic cell nuclei. Here we give a brief account of how these experimental advantages can be exploited to analyze the splicing processin vivo.In multi-intron pre-mRNAs, spliceosomes assemble and splicing is initiated cotranscriptionally for all introns. Intron excision may, however, occur mainly co- transcriptionally or mainly posttranscriptionally depending on the position of each intron in relation to the remaining transcription time and intron-specific efficiencies of excision. As measured for the U2 snRNP and an SR protein, 10–15% of the spliceosomal components are bound to pre-mRNA at active gene loci at a given moment, while the majority of the spliceosomal components are present in the nucleoplasm. A continuous redistribution of the spliceosomal components takes place in the nucleus as a result of a close coupling between transcription and spliceosomal assembly.