Assembly and disassembly of spliceosomes along a specific pre‐messenger RNP fiber.

Transcriptionally active Balbiani ring (BR) genes in the salivary glands of the dipteran Chironomus tentans were studied by immunoelectron microscopy to establish the distribution of spliceosome components along a specific pre‐messenger ribonucleoprotein (pre‐mRNP) fiber. The BR genes are 35‐40 kb in size with three introns close to the 5′ end and one close to the 3′ end; a very large middle portion lacks introns. As a rule the 5′ introns are spliced concomitant with transcription in the promoter proximal third of the gene, while the 3′ intron is spliced post‐transcriptionally. The BR genes with growing pre‐mRNPs were visualized in situ, while completed and released pre‐mRNPs were isolated from the nucleoplasm and studied unfolded on a grid surface. An anti‐snRNP antibody (Y12) bound mainly to the promoter proximal third of the BR gene (86%) and only to a minor extent to the middle and distal thirds (7 and 7% respectively). An antibody to an hnRNP protein reacted with the proximal, middle and distal regions to an increasing extent (17, 38 and 45% respectively), reflecting the increase in size of the growing transcription product. In the nucleoplasmic pre‐mRNP particle only one end of the RNP fiber was labeled by Y 12, presumably the 3′ end; the anti‐hnRNP antibody decorated the entire RNP fiber. Thus, the snRNPs do not associate along the whole pre‐mRNP fiber but rather bind to the 5′ and 3′ ends, i.e. the regions containing the introns. The results also imply that the spliceosomes both assemble and disassemble rapidly on the pre‐mRNP fiber.