ASSEMBLY AND DISASSEMBLY OF SPLICEOSOMES ALONG A SPECIFIC PREMESSENGER RNP FIBER

Transcriptionally active Balbiani ring (BR) genes in the salivary glan ds of the dipteran Chironomus tentans were studied by immunoelectron m icroscopy to establish the distribution of spliceosome components alon g 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 t he promoter proximal third of the gene, while the 3' intron is spliced post-transcriptionally. The BR genes with growing pre-mRNPs were visu alized in situ, while completed and released pre-mRNPs were isolated f rom the nucleoplasm and studied unfolded on a grid surface. An anti-sn RNP 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 thir ds (7 and 7% respectively). An antibody to an hnRNP protein reacted wi th the proximal, middle and distal regions to an increasing extent (17 , 38 and 45% respectively), reflecting the increase in size of the gro wing transcription product. In the nucleoplasmic pre-mRNP particle onl y 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 snR NPs 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 resu lts also imply that the spliceosomes both assemble and disassemble rap idly on the pre-mRNP fiber.